Preserve AskPrimary/AnswerPrimary for neoctl/neoadmin exchange only.
Fixes several race conditions during election.

Otherwise, in the case of an unexpected packet, an incorrect message id
would be used.
Also, refuse setting a None peer_id on connection.

This includes an API change of Node.isIdentified, which now tells whether
identification packets have been exchanged or not.
All handlers must be updated to implement '_acceptIdentification' instead of
overriding EventHandler.acceptIdentification: this patch only does it for
StorageOperationHandler

Roughly once every 40 tries on ClusterTests.testElectionWithManyMasters I
get a "port already in use" because runner assigned two master to the same
port.

MasterHandler contains only server-side handlers.

Also, factorise code to be executed when connection is established.

Also, drop captain-obvious comments.

Also, make test_ClientConnection_writable2 test what it says it tests.

Replication is also fully reimplemented:
- It is not done anymore on whole partitions.
- It runs at lowest priority not to degrades performance for client nodes.

Schema of MySQL table is changed to optimize storage layout: rows are now
grouped by age, for good partial replication performance.
This certainly also speeds up simple loads/stores.