server: add a new control CTDB_CONTROL_TRANS3_COMMIT

server: add a new control CTDB_CONTROL_TRANS3_COMMIT

This is a simplified version of the trans2 commit control:
It just rolls out the marshall buffer to all active nodes.

It is the main ctdbd part of the re-implementation of the
persistent transactions. The client code is changed to
take a global lock to start a transactions and store into
the marshal buffer instead of writing to the local tdb
under a local transaction.

The old transaction implementation is going to be
removed in a later commit.

Michael