* cleanup: go vet and golint run
Various cleanups trickered by go vet and golint.
* Fix tests and lowercase all errors
Lowercase all errors, some tests in kubernetes use errors from
kubernetes which do start with a capital letter.
Check message for expired sig and don't cache those.
Aside: This hack of caching entire messages is probably something we
should stop doing at some point in the future and do this on a per RRset
basis.
Fixes#367#635