This really fixes the bug described in
commit 40bac312,
which could probably be reverted. It only reduced the probability of failure.

What happened is that the second conflict on 'a' for t3 what first reported by
an answer to first store with:
- a base serial at which a=0
- a conflict serial at which a=7
However, the cached data is not 8 anymore but 12, since a second store already
occurred after the first conflict (reported by the other storage node).

When this conflict was resolved before receiving the conflict for second store,
it gave:

  resolve(old=0, saved=7, new=12) -> 19

instead of:

  resolve(old=4, saved=7, new=12) -> 15

(if we still had the data of the first store, we could also do
  resolve(old=0, saved=7, new=8)
 but that would be inefficient from a memory point of view)

The bug was difficult to reproduce. testNotifyReplicated had to be run many
many times before that race conditions trigger it. The test was changed to
enforce some of them, and the above scenario now happens almost always.

- atomic write to disk to avoid corruption
- update when the address changes (not only when a node is removed/added)

This fixes 2 issues:
- Because neoctl connects to admin nodes without requesting identification,
  the protocol version was not checked, which could even be dangerous
  (think of a user asking for information, but the packet sent by neoctl
  could be decoded as a packet to alter data, like Truncate).
- In case of mismatched protocol version, the error was not loggued on the
  node that initiated the connection.

Compatibility is handled as follows:
- For an old node receiving data from a new node, the 2 high order bytes of the
  packet id, which is always 0 for the first packet, is decoded as the packet
  code. Packet 0 has never existed, which results in PacketMalformedError.
- For a new node receiving data from an old node, the id of the first packet,
  which is always 0, is decoded as the version, which results in a version
  mismatch error.

This new protocol also guarantees that there's no conflict with SSL.

For simplification, the packet length does not count the header anymore.

When using network byte order ('!'), the size of struct items is independant of
the platform. They have never changed from one version of Python to another.

The election is not a separate process anymore.
It happens during the RECOVERING phase, and there's no use of timeouts anymore.

Each master node keeps a timestamp of when it started to play the primary role,
and the node with the smallest timestamp is elected. The election stops when
the cluster is started: as long as it is operational, the primary master can't
be deposed.

An election must happen whenever the cluster is not operational anymore, to
handle the case of a network cut between a primary master and all other nodes:
then another master node (secondary) takes over and when the initial primary
master is back, it loses against the new primary master if the cluster is
already started.

In order to do that correctly, this commit contains several other changes:

When connecting to a primary master, a full node list always follows the
identification. For storage nodes, this means that they now know all nodes
during the RECOVERING phase.

The initial full node list now always contains a node tuple for:
- the server-side node (i.e. the primary master): on a master, this is
  done by always having a node describing itself in its node manager.
- the client-side node, to make sure it gets a id timestamp:
  now an admin node also receives a node for itself.

This keeps the connection fully functional when a handler raises an exception.

Commits like 7eb7cf1b
("Minimize the amount of work during tpc_finish") dropped what was done in
commit 07b48079
("Ignore some requests, based on connection state") to protect request handlers
when they respond.

This commit fixes this in a generic way.

The initial intention was to rely on stable sorting when several events have
the same key. For this to happen, sorting must not continue the comparison with
the second item of events.

This could lead to data corruption (conflict resolution with wrong base):

  FAIL: testNotifyReplicated (neo.tests.threaded.test.Test)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "neo/tests/threaded/__init__.py", line 1093, in wrapper
      return wrapped(self, cluster, *args, **kw)
    File "neo/tests/threaded/test.py", line 2019, in testNotifyReplicated
      self.assertEqual([15, 11, 13, 16], [r[x].value for x in 'abcd'])
    File "neo/tests/__init__.py", line 187, in assertEqual
      return super(NeoTestBase, self).assertEqual(first, second, msg=msg)
  failureException: Lists differ: [15, 11, 13, 16] != [19, 11, 13, 16]

  First differing element 0:
  15
  19

  - [15, 11, 13, 16]
  ?   ^

  + [19, 11, 13, 16]
  ?   ^

'aborted' could appear twice.

zodburi[1] provides a way to open ZODB storages by URL/URI. It already
has support for file:// zeo:// zconfig:// memory:// etc schemes out of
the box and third-party-to-ZODB storages can add support for their
schemes via providing zodburi.resolvers entrypoint.

For example relstorage and newtdb do this.

Let's also teach NEO to open itself via neo:// URI schema.

[1] http://docs.pylonsproject.org/projects/zodburi
[2] https://github.com/zodb/relstorage/blob/2.1a1-15-g68c8cf1/relstorage/zodburi_resolver.py
[3] https://github.com/newtdb/db/blob/0.5.2-1-gbd36e90/src/newt/db/zodburi.py

Commit 58d0b602 didn't fix the issue completely.
Storage space can be freed with --repair option.

This adds an expectedFailure test.

This is a follow up of commit 64afd7d2,
which focused on read accesses when there is no transaction activity.

This commit also includes a test to check a simpler scenario that the one
described in the previous commit.

Commit ad43dcd3 should have bumped it as well.

Unused but it is likely to be useful in the future.

The bug could lead to data corruption (if a partition is wrongly marked as
UP_TO_DATE) or crashes (assertion failure on either the storage or the master).

The protocol is extended to handle the following scenario:

    S                                    M
    partition 0 outdated
      <-- UnfinishedTransactions ------>
    replication of partition 0 ...
    partition 1 outdated
      --- UnfinishedTransactions ...
    ... replication finished
      --- ReplicationDone ...
                                         tweak
      <-- partition 1 discarded --------
                                         tweak
      <-- partition 1 outdated ---------
          ... UnfinishedTransactions -->
          ... ReplicationDone --------->

The master can't simply mark all outdated cells as being updatable when it
receives an UnfinishedTransactions packet.

After an attempt to read from a non-readable, which happens when a client has
a newer or older PT than storage's, the client now retries to read.

This bugfix is for all kinds of read-access except undoLog, which can still
report incomplete results.

This revert commit bddc1802,
to fix the following storage crash:

  Traceback (most recent call last):
    ...
    File "neo/lib/handler.py", line 72, in dispatch
      method(conn, *args, **kw)
    File "neo/storage/handlers/master.py", line 44, in notifyPartitionChanges
      app.pt.update(ptid, cell_list, app.nm)
    File "neo/lib/pt.py", line 231, in update
      assert node is not None, 'No node found for uuid ' + uuid_str(uuid)
  AssertionError: No node found for uuid S3

Partitition table updates must also be processed with InitializationHandler
when nodes remain in PENDING state because they're not added to the cluster.