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