* 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
Adds a middleware to enable tracing with OpenTracing/OpenZipkin.
Enabling tracing will have a large impact on performance so it is
not advisable in production.
