agent: Also evict cached items via a timer.

* agent/cache.c (agent_cache_housekeeping): New func.
* agent/gpg-agent.c (handle_tick): Call it.

This change mitigates the risk of having cached items in a post mortem
dump.

• GnuPG-bug-id: T3829
• Signed-off-by: Werner Koch <wk@gnupg.org>