ctdb-server: Implement CTDB_CONTROL_START_IPREALLOCATE

ctdb-server: Implement CTDB_CONTROL_START_IPREALLOCATE

Trigger a "startipreallocate" event, but only if in RUNNING runstate.
"startipreallocate" is intended to allow an NFS server to be put into
grace on all nodes before any locks are released as part of releaseip
during failover.  If node A is leader and initiates a takeover run
then node B may be connected/active but may not have completed
startup.  In this case, the attempt to put NFS-Ganesha into grace on
node B will fail, startipreallocate will fail, and the node will be
banned.

Signed-off-by: Vinit Agnihotri <vagnihotri@ddn.com>
Reviewed-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Volker Lendecke <vl@samba.org>