Add a cache of recently seen addresses.

This maintains a cache file for each host we have communicated with, either
via TCP or UDP. The cache is used when trying to make outgoing connections,
and is updated whenever a successful TCP or UDP connection is established.
Up to 8 addresses are stored in the cache.

Currently, the cache is stored in /etc/tinc/NETNAME/cache. The directory
has to be manually created to opt in to this feature for now.