gpg: Improve speed of --check-sigs and --lish-sigs.

* g10/keydb.c (kid_list_t): New.
(kid_not_found_table, n_kid_not_found_table): New.
(kid_not_found_p, kid_not_found_insert, kid_not_found_flush): New.
(keydb_insert_keyblock): Flush the new cache.
(keydb_delete_keyblock): Ditto.
(keydb_update_keyblock): Ditto.
(keydb_search): Use the new cache.
(keydb_dump_stats): New.
* g10/gpg.c (g10_exit): Dump keydb stats.

What we do here is to keep track of key searches by long keyids (as
stored in all signatures) so that we do not need to scan the keybox
again after we already found that this keyid will result in
not-found. As soon as we change gpg to run as a co-process we should
store this table per session because other instances of gpg may have
updated the keybox without us knowing.

On a test ring with

gpg: 94721 good signatures
gpg: 6831 bad signatures
gpg: 150703 signatures not checked due to missing keys
gpg: 5 signatures not checked due to errors
gpg: keydb: kid_not_found_table: total: 14132

this new cache speeds a --check-sigs listing up from 28 minutes to
less than 3 minutes.

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