* dnstest: add multirecorder
This adds a new recorder that captures all messages written to it. This
can be useful when, for instance, testing AXFR which can write muliple
messages back to the client.
* docs
Add a full test server impl in this new package + tests. Move
dnsrecorder into this package as well and finish up the commented out
tests that were left in the old dnsrecorder package.
Update all callers and tests.
