dirmngr: Avoid need for hkp housekeeping.

* dirmngr/ks-engine-hkp.c (host_is_alive): New function.  Test whether
host is alive and resurrects it if it has been dead long enough.
(select_random_host, map_host, ks_hkp_mark_host): Use host_is_alive
instead of testing hostinfo_t->dead directly.
(ks_hkp_housekeeping): Remove function, no longer needed.
* dirmngr/dirmngr.c (housekeeping_thread): Remove call to
ks_hkp_housekeeping.

Rather than resurrecting hosts upon scheduled resurrection times, test
whether hosts should be resurrected as they're inspected for being
dead. This removes the need for explicit housekeeping, and makes host
resurrections happen "just in time", rather than being clustered on
HOUSEKEEPING_INTERVAL seconds.

Gbp-Pq: Topic dirmngr-idling
Gbp-Pq: Name dirmngr-Avoid-need-for-hkp-housekeeping.patch

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>