SERVER-15399 Implementation of NetworkInterface that uses its own thread and connection pools.

Behavior is believed to match that of ScopedConn.
SERVER-14447 Implementation of processReplSetReconfig for ReplicationCoordinatorImpl.
SERVER-14561 Set heartbeat "from" field and "version" fields like v2.6 when the node has no config.
Merge remote-tracking branch 'upstream/master'