Summary:

* requestor: fixed connected UDP sockets (not used in authoritative)
* requestor: connects only if layers generate an answer (this saves connect() if a query can be satisfied from cache)
* namedb: track full mapsize (ENOSPC)
* namedb: memory reservation on insert (saves one alloc and memset)
* packet: don't prealloc rrsets, allocate them from the packet mempool (not usable in resolver, as each request usually needs multiple queries)

Dockerfiles for Knot DNS & tests

Automatic build is in the registry https://registry.hub.docker.com/u/cznic/knot/

See merge request !328

Conflicts:
	scripts/Dockerfile

this is required, as some requests may be satisfied without needing
the connection, or fail in which case a connection is superfluous

Conflicts:
	scripts/Dockerfile

image)

this comes with some extra CPU cost, however:
- parsed packets are prealloc'd based on header information, so no
  reallocs
- constructed packets are alloc'd by steps
- packet construction / copy is  cheaper now
- doesn't matter when the mm backend is mempool anyway

the rationale for this change is the use in resolver and in smaller
installations