Live Merge: Simplify pivot flow

The pivot part of a Live Merge can be made a synchronous operation which
simplifies our flows.  We're already doing cleanup in a thread so just
do the pivot and wait for it (very short) to complete and then continue
directly to the volume chain sync part of the cleanup.

This is much simpler than doing the pivot and waiting another sample
interval to try the sync portion.

Change-Id: I5e6a9ea7096b5d81f418754d411f135450bf572e
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31849
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>