dirmngr: Add FLUSHCRLs command

Summary:

• dirmngr/crlcache.c (crl_cache_flush): Also deinit the cache.
• dirmngr/server.c (hlp_flushcrls, cmd_flushcrls): New.

(register_commands): Add FLUSHCRLS.

This allows it to flush the CRL cache of a running dirmngr
server. This can be useful to debug / analyze CRL issues.

Differential Revision: https://dev.gnupg.org/D469

• GnuPG-Bug-Id: T3967
• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>