ORNL-28: remove lock contention in reconnecting

target_handle_connect() used to grab obd::obd_recovery_task_lock to update
obd_next_recovery_transno and obd_connected_clients. In this patch, I revised
this piece of code by:
- modify obd_connected_clients to cfs_atomic_t
- grab obd_recovery_task_lock only if ocd_transno is less than
  obd_next_recovery_transno

In this way, target_handle_connect() don't grab any global lock.

Change-Id: I971897de82566e239fe750cf95f2c3f1646325bc
Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1293
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>