LU-570: Add function to find connect uuid by nid

LU-570: Add function to find connect uuid by nid

In this patch, two functions are added:
- class_find_uuid(), find conn uuid by peer nid
- client_import_find_conn(), find a conn uuid in import connection list

Also, a code cleanup is performed.

Change-Id: I50e8e9392a39ef78719504cf083c0c22f5d39dcb
Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1189
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>