gpgsm: Use a cache to speed up parent certificate lookup.

* sm/gpgsm.h (COMPAT_NO_CHAIN_CACHE): New.
(struct cert_cache_item_s, cert_cache_item_t): New.
(struct server_control_s): Add parent_cert_cache.
* sm/gpgsm.c (compatibility_flags): Add "no-chain-cache".
(parent_cache_stats): New.
(gpgsm_exit): Print the stats with --debug=memstat.
(gpgsm_deinit_default_ctrl): Release the cache.
* sm/certchain.c (gpgsm_walk_cert_chain): Cache the certificates.
(do_validate_chain): Ditto.

This gives another boost of 30% (from 6.5 to 4.0 seconds in the test
environment with ~1000 certs). do_validate_chain actually brings us
the speedup becuase the gpgsm_walk_cert_chain is not used during a key
listing. For the latter we actually cache all certificates because
that was easier.

Adjusted for 2.2:

• Add gpgsm_deinit_default_ctrl
• Remove ctrl arg from keydb_new

• GnuPG-bug-id: T7308