rpl_semi_sync_master_wait_sessions was reset by FLUSH STATUS,
which could cause the master fail to wake up waiting sessions and
result in master timeout waiting for slave reply.

rpl_semi_sync_master_wait_session should not be reset, this 
problem is fixed by this patch.


plugin/semisync/semisync_master_plugin.cc:
  Change wait_sessions from SHOW_LONG back to SHOW_FUNC so that it will not be reset by FLUSH STATUS.