Add a cache-snapshot-ttl configuration variable

Add a cache-snapshot-ttl configuration variable

This can be used to specify the TTL for snapshots. Snapshots are usually
static and do not ever change. On the other hand, tarball generation is
CPU intensive.

One use case of this setting (apart from increasing the lifetime of
snapshot cache slots) is caching of snapshots while disabling the cache
for static/dynamic HTML pages (by setting TTL to zero for everything
except for snapshot requests).

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>