Noticed because gitweb syntax colouring disliked it.

This fixes a regression in commit 4720ba33.

Inspired from "mdadm assemble --no-degraded" behaviour, which was brought
up during a discussion on linux-raid mailing list about split-brain
situation detection and avoidance.

PortAllocator is broken by design and the only proper solution to run a NEO
functional test without race condition with another software opening a random
port is to use ... unix sockets.

Keyword arguments are kept internally by the handler switcher, which passes
them to the handler when the peer answers.

This will be required by new replicator, so that 'setUnfinishedTIDList'
receives the list of outdated partitions at the time the
'AskUnfinishedTransactions' packet was sent.

This comment just described the whole purpose of the code, so there is no
XXX there.

If start() fails here, it fail in the next test too (and any test until
stop gets called).

For easier understanding of answerLastIDs' final else block.

Condition is true from the first iteration, as we just filled
unconnected_master_node_set.

Also, fix stupid error in error handling path.

Fixes a crash when a node aborts a connection and then detecting it as
closed: it would attempt to unregister from event manager twice.

This increases cluster on-line flexibility, allowing new masters to be
added without needing a restart of all masters.
Downside is that more responsibility is pushed to cluster admins: they must
update node configurations so they contain enough master nodes to allow
them to join cluster after a restart.