s3:dbwrap_ctdb: start rewrite of transactions using the global lock (g_lock)

s3:dbwrap_ctdb: start rewrite of transactions using the global lock (g_lock)

This simplifies the transaction code a lot:

* transaction_start essentially consists of acquiring a global lock.

* No write operations at all are performed on the local database
  until the transaction is committed: Every store operation is just
  going into the marshall buffer.

* The commit operation calls a new simplified TRANS3_COMMIT control
  in ctdb which rolls out thae changes to all nodes including the
  node that is performing the transaction.

Michael
(cherry picked from commit 16bc6ba2268e3660d026076264de8666356e00bf)
(cherry picked from commit c2354aa193617ffc3e4036fe0b9034c2664901fd)