XDCR: should have connection count metric and better connection logging

At present, it is difficult to say for certain how many connections are active because connection count depend on many factors e.g replication count, target node count, replication configuration etc

So following things should be done:

1. Add metric in XDCR to track active connections to remote memcached
2. Add logging when creating and closing connections tagged with relevant ID (e.g Xmem Nozzle ID)