gencache: Remove transaction-based tdb

At more than one large site I've seen significant problems due to
gencache_stabilize. gencache_stabilize was mainly introduced to
survive machine crashes with the cache still being in place. Given
that most installations crash rarely and this is still a cache, this
safety is overkill and causes real problems.

With the recent changes to tdb, we should be safe enough to run on
completely corrupted databases and properly detect errors. A further
commit will introduce code that wipes the gencache.tdb if such a
corruption is detected.

There is one kind of corruption that we don't properly handle:
Orphaned space in the database. I don't have a good idea yet how to
handle this in a graceful and efficient way during normal operations,
but maybe this idea pops up at some point.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>