gpg: Keep a lock during the read-update/insert cycle in import.

* g10/keydb.c (keydb_handle): New field 'keep_lock'.
(keydb_release): Clear that flag.
(keydb_lock): New function.
(unlock_all): Skip if KEEP_LOCK is set.
* g10/getkey.c (get_keyblock_byfprint_fast): Call keep_lock if
requested.

That change is straightforward. It helps to avoid the race condition
that another gpg process inserts a key while the first process is
between the search and the insert.

A similar change is due for gpgsm.

Note that the key edit operations may still suffer from a race.

• GnuPG-bug-id: T3446