LU-580: update mgc llog process code

LU-580: update mgc llog process code

This patch does the following work:
1. fix a bug about setting mgc lock callback data;
2. define log instance types;
3. simplify and cleanup mgc_requeue_thread code

Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Change-Id: Ic8d25eda9f0a7884b4a8cbeccc89c1e66c19f0ff
Reviewed-on: http://review.whamcloud.com/1198
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>