gpg: Remove tofu database format "split".

* g10/options.h (struct opt): Remove field tofu_db_format.
* g10/gpg.h (server_control_s): Add fields tofu.batch_update_ref and
tofu.batch_update_started.
* g10/gpg.c (parse_tofu_db_format): Remove.
(main): Make option --tofu-db-format obsolete.
* g10/tofu.c: Major rework.  Remove the pretty complicated and slower
split format and with that all the caching.  Use the dbs struct
directly.  Move global vars for batch update into CTRL.  Change
calling conventions of some function to take CTRL or DBS pointers
instead of  the former low-level database pointer.

The split database format might have been nice for use with Unison but
it bypasses the concept of a relational database by doing parts of
this itself and also risking deadlocks. Working with the Tofu
database for debugging or experiments is also not possible with parts
of the database logic implemented in gpg.

The Tofu support is quite new and we can assume that it is not in real
use now. Thus we better remove that now so that we do not need to
maintain it for all future.

• Signed-off-by: Werner Koch <wk@gnupg.org>