I considered a couple of different factorings, that is
* Implementing the fallback as a Lookup
* Having a server struct that held the resolv.conf and methods for
handling queries
but ultimately, this is simple and works.
* Put prefixes (including msg IDs) in front of log messages to
distinguish them
* Failed lookups get logged under Info as per issue #311
* More accurate messages as per issue #311
This separates Zone into the DB ops and the Lookup ops, and implements
the latter for MDNSClient so it can be used just like the zonedb.
A notable change is that Lookup methods only ever return one result,
while MDNSClient.SendQuery could in theory return multiple results
(though it wouldn't, because no server would supply more than one
result in a response, and it stops waiting after getting one response
..).
ReverseLookupLocal is implemented even though no MDNSServer will
answer at present (hence no test yet).
Lastly, the handler (creating) functions can now be given a list of
Lookup implementations to try in order.
Michael Bridgen