libceph: cleanup old messages according to reconnect seq

when reopen a connection, use 'reconnect seq' to clean up
messages that have already been received by peer.

Link: http://tracker.ceph.com/issues/18690


Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>