* 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
* Switches out Unhealthy bool for OkUntil timestamp
* Make sure servers are healthy forever if there are no health checks
* Moves health check off into a go routine to avoid blocking conditions
* Improved logging info
* Fixes initial date
* Fixes health checking; alters tests to adapt to async health checking
* Moves future variable into static upstream and populates it in more places
* Restores silencing of stdout during testing
* Restores silencing of stdout during testing
* keeps check url string once built
* Removes debug message
* uses zero value to signal no checking; reduces in-mutex code to a fetch
* checkpoint
* Pass context through ServeDNS, enable gRPC tracing
* Fix types and make tracer available to proxy. go fmt
* Fix imports
* Use the DoNotStartTrace option
* Change to SpanFilter from DoNotStartTrace
* Use new name (IncludeSpan)
* Final names
* Add tests; fix possible client/conn leaks in grpc
* go fmt
