mds: avoid sending duplicated discovers during recovery

mds: avoid sending duplicated discovers during recovery

If MDS just entered the rejoin state, it should not kick discovers
because the discovers were just sent. Similarly, if MDS just entered
the clientreplay state, it should not call MDS::handle_mds_recovery()
because MDS::recovery_done() has already recovered the table server.

Also make MDCache::handle_mds_recovery() not wake the discover waiters
up. Because the MDCache::kick_discovers re-sends the discovers, their
replies will wake the discover waiter up.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>