agent: Serialize access to passphrase cache.

agent: Serialize access to passphrase cache.

* agent/cache.c (encryption_lock): Remove.
(cache_lock): New.  Now, we have coarse grain lock to serialize
entire cache access.
(initialize_module_cache): Use CACHE_LOCK.
(init_encryption, new_data): Remove ENCRYPTION_LOCK.
(agent_flush_cache, agent_put_cache, agent_get_cache): Lock the cache.

--

GnuPG-bug-id: 3027
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
(cherry picked from commit ebe12be034f052cdec871f0d8ad1bfab85d7b943)

Gbp-Pq: Name 0064-agent-Serialize-access-to-passphrase-cache.patch