This is currently not an issue because the 'time.sleep(1)' in iterateForObject
(storage) and _connectToPrimaryNode (master) leave enough time. What could
happen is a new connection attempt for a node that already has a connection
(causing a failure assertion in Node.setConnection).

This could happen if a file descriptor was reallocated by the kernel.

With this patch, the epolling object is not awoken every second to check
if a timeout has expired. The API of Connection is changed to get the smallest
timeout.

This is a prerequisite for tickless poll loops.

This mainly changes several methods to lock automatically instead of asserting
that the caller did it. This removes any overhead for non-MT classes, and
the use of 'with' instead of lock/unlock methods also simplifies the API.

shutdown is implicit because we don't duplicate sockets.

- For all threads except the main one, the id is displayed instead of the name,
  because the latter is not always unique.
- Outputs may be interlaced by concurrent thread, so tracebacks are also
  prefixed by their idents.

When the connection to the primary master node is lost, the node manager
does not have anymore a reliable list of running nodes, so iterateForObject()
must not retry any cell.

Since transactions have metadata like a description, it may not be useless
to allow them. But the behaviour of FileStorage is to silently drop them,
so we may have to do the same in the future.

An application that is not supposed to commit empty transactions should write
its own unit test to prevent this.

This happened between storage nodes of different clusters because they're not
informed about their state, e.g. a dead upstream storage node.

In any case, logs were flooded at 100% cpu usage.

Binding a port actually does not reserve it. And on the other side, a bound
socket can't be bound again. So it could bind a port twice, warn about this,
and then raise EINVAL when trying to bind again.

Apart from this, the global lock didn't even prevent conflict with another
NEO test run when tests restart nodes. So better keep it simple.

Limiting the size of data.value column to 16 MB saves 1 byte by switching
to MEDIUMBLOB, and it avoid the need of big redo logs in InnoDB.