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.

See http://bugs.python.org/issue13361 ("getLogger does not check its argument").

Custom ctypes wrapper for epoll is not needed with python 2.6+.
Also, do not retry upon temporary error: it is valid to handle no packet,
and retrying the poll call would increase overall timeout without easy fix.
Also, remove meaningless frozensets (leftovers from an old bug's symptoms).

Also simplify some code by using some new Python syntax.

This hask is mainly for performance tests, which should be run on a physical
machine that doesn't do anything else.