Implement health.Healther in erratic and kubernetes plugin. The
kubernetes' healtcheck is only performed on startup - i.e. turn
healthy after the initial loading.
Erratic follow the drop count: every query%drop turns the healthcheck
unhealthy.
Fixes: #985
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.
* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
