Use QNAME from the packet as a zone name in log messages for incoming
requests. The zone name need not be set in the query_data structure.

Requestor refactoring

- move requestor from libknot to server
- make requestor use just one layer
- remove queuing from requestor
- remove concept of overlays

See merge request !539

The processing context is now initialized once per handler
instead of once per query.

The overlay is currently used with exactly one layer, thus
we can use the layer directly.