glusterd/snapshot: brick_start shouldn't be done from child thread

When creating a volume snapshot, the back-end operation 'taking a
lvm_snapshot and starting brick' for the each brick
are executed in parallel using synctask framework.

brick_start was releasing a big_lock with brick_connect and does a lock
again.
This will cause a deadlock in some race condition where main-thread waiting
for one of the synctask thread to finish and
synctask-thread waiting for the big_lock.

Solution is not to start_brick from from synctask

Change-Id: Iaaf0be3070fb71e63c2de8fc2938d2b77d40057d
BUG: 1100218
Signed-off-by: Vijaikumar M <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/7842
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>