ctdb_ltdb_store_server: implement fastpath vacuuming deletion based on VACUUM_MIGRATE...

ctdb_ltdb_store_server: implement fastpath vacuuming deletion based on VACUUM_MIGRATED flag.

When the record has been obtained by the lmaster as part of the vacuuming-fetch
handler and it is empty and never been migrated with data, then such records
are deleted instead of being stored. These records have automatically been
deleted when leaving the former dmaster, so that they vanish for good when
hitting the lmaster in this way. This will reduces the load on traditional
vacuuming.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>