dbwrap ctdb: release the lock before calling ctdbd_persistent_store()

in the persistent db_ctdb_store operation.

This is to prevent deadlocks in db_ctdb_persistent_store().

There is a tradeoff: Usually, the record is still locked
after db->store operation. This lock is usually released
via the talloc destructor with the TALLOC_FREE to
the record. So we have two choices:

- Either re-lock the record after the call to persistent_store
  or cancel_persistent update and this way not changing any
  assumptions callers may have about the state, but possibly
  introducing new race conditions.

- Or don't lock the record again but just remove the
  talloc_destructor. This is less racy but assumes that
  the lock is always released via TALLOC_FREE of the record.

I choose the first variant for now since it seems less racy.
We can't guarantee that we succeed in getting the lock
anyways. The only real danger here is that a caller
performs multiple store operations after a fetch_locked()
which is currently not the case.

Michael
(This used to be commit d004c9a7281d2577c3ba2012c8f790cc198ea700)

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