Complete DNSSEC cache.

Update readme, add new flag, etc.
Also: prepare for response cache which will make use of the same
underlaying cache implementation.
Allow zero capacity
Rename Size to Capacity, because that is what it is
Set the ttl in cache
Make DNSSEC cache work properly

Get the expiration date from the data and check if that is still
valid 24 hours from now. The exact same code (with minor changes)
can be used for the message cache.