ctdb: Make TDB_SEQNUM work synchronously with ctdb

ctdb: Make TDB_SEQNUM work synchronously with ctdb

Old war story completely from memory, I could not find the commit that
introduced TDB_SEQNUM so far...:

Back in the days when ctdb was initially developed, TDB_SEQNUM's only
user was the notify.tdb that held one huge record for all notify
records. With that use case in mind it made perfect sense to keep the
SEQNUM stable locally, sacrificing precision. By now notify.tdb is
long gone, an the only user of TDB_SEQNUM right now is brlock.tdb,
which contains special case code for the imprecise ctdb implementation
of TDB_SEQNUM.

With this commit, that special code can go: The TDB_SEQNUM will also
increment when just the DMASTER header field changes, indicating to
smbd that someone else might have changed the record. This will of
course increase the SEQNUM frequency, but it should not increase the
load on ctdb: If you look at the brlock.c workaround, it just does not
do the caching that is possible with precise TDB_SEQNUMs working.

How did I get here? I want to move brl_num_read_oplocks() from
brlock.tdb into locking.tdb, and for that I need precise TDB_SEQNUMs
for locking.tdb.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri May 24 00:42:17 UTC 2019 on sn-devel-184