LU-1166 recovery: don't leak a connected client counter.

target_handle_connect vs client eviction race may leak a
connected client counter and some evicted clients will counted twice.

Xyratex-bug: MRP-451

Change-Id: Id35baae16ae09bc3647d089b45b95e967582d09b
Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-on: http://review.whamcloud.com/2255
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>