more dnssec tests
Cache whole packets
Put signed msgs in the cache under a diff. key

Cache sign message under a different cache key, so that signed and unsigned
replies do not clash with each other. Cache complete messages.

Also fix the double signing and unnessecary adding of NSEC3 records in DNSSEC
replies.

Also fix message too large when replying from cache, by bluntly setting the TC
bit.

Closes #80