Make all transactions nested in ldb. The current samba4 code expects this

Make all transactions nested in ldb. The current samba4 code expects this
behavior anyway, and given we can only have one transaction active per
ldb context this is the only sane model we can support.

Fix ldb_tdb transactions, we could return back with an error with neither
committing nor canceling the actual tdb transaction in some error paths
within the ltdb commit and cancel transaction paths.

Added also some debugging to trace what was going on.