Improve msg.Len() accuracy

Currently msg.Len() overshoots a _lot_ when compression is
enabled. The main problem is that once a part of a domain is found to
be compressed, the whole domain is _not_ added back to compression
dictionary.

That means if we once cache ".microsoft.com" we won't cache
"blah.microsoft.com".

Ammended a bit: put the test in dns_test.go
Merge branch 'len'

Cleanup the rest and add the test.
Document and disable the test