* 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
* core: hide registerHandler
Remove RegisterHandler and just make it implicit when we look at the
handler compilation step.
* Rename GetHandler to just Handler
Update callers and make auto check Hander in OnStartup.
* Up test coverage in erratic
* up test coverage
This middleware allows playing with responses. Only one type is
implemented: it allows you to drop queries. I.e. withhold the response
from the client.
