* 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
* Implements tracing in the native gRPC server
* Undo some unnecessary changes
* Properly revert trace/setup.go this time
* Some very very basic tests
* Remove warning for non-Trace middleware