glusterd: Release big-lock after log-rotate handler returns

... so that subsequent volume commands don't block waiting forever,
for the lock to be released.

Change-Id: I24b5ec47f6982900ab74ff1b492d523f31ecfb7f
BUG: 1022055
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/6122
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>