glusterd: Avoided deadlock in single node cluster, glusterd restart

In a single node cluster, it is possible to deadlock on the "big
lock", while restarting bricks. In glusterd_restart_bricks, we perform a
glusterd_brick_connect, where we release the big lock in anticipation
that glusterd_brick_rpc_notify could run in the same C stack (and
deadlocking). So, in the restart code path, we could unlock before we
have performed a lock on the big lock.

To fix this, we need to take the big lock in the
glusterd_launch_synctask 'thread' as well.

Change-Id: I1abea1ca82b55c784b8a810a8194f254b32b1dcc
BUG: 948686
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/4837
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>