Merge branch 'resolver_improvements' into 'master'

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)