Periodical Anti-entropy with random neighbor

As stated in http://concerto.xwiki.com/xwiki/bin/view/Main/AntiEntropie , Anti-entropy should be run periodically with a random neighbour to update the local log with lost messages.

This should be materialized as a thread that runs as long as the peer is connected to a group (can send/receive messages) and is connected to the content provider (can apply eventually received messages from the anti-entropy reply).

While the peer is not connected to a group or to the content provider, keep the thread stopped.