Main reason is that it's difficult to know in advance which side really closes
the connection. Network events can be chaotic and this could lead to many race
conditions.
Thus, handler can be used to update any database that is somewhat redundant
to the connection status, i.e. node status usually. Safely and less duplicated
code.

This change is motivated by recurrent random failures during election.
An example of race condition was that 2 fully connected master could close the
extra connection (the primary -> secondary one) at the same time.

In order to stabilize lower-level code and start with reliable election process,
code has also been simplified to not care about node states. All connections
without exception are closed at the end of the election and states are then
updated 1 by 1 by identification handler.
Note that during election, there may be 2 connection per node, which makes
difficult to update node states by connectionFailed/connectionClosed events.

timeoutExpired & peerBroken are dropped as they are unused for the moment.
A new API should be designed so that connectionClosed know the reason of the
close.
BROKEN state becomes unused.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2732 71dcc9de-d417-0410-9af5-da40c76e7ee4