global: move to context (#1699)

* global: move to context

Move from golang.org/x/net/context to std lib's context.

Change done with:

for i in $(grep -l '/context' **/*.go); do sed -e 's|golang.org/x/net/context|context|' -i $i; echo $i; done
for i in **/*.go; do goimports -w $i; done

* drop from dns.pb.go as well

plugin/trace/trace.go

@@ -11,11 +11,12 @@ import (
 	// Plugin the trace package.
 	_ "github.com/coredns/coredns/plugin/pkg/trace"
 
+	"context"
+
 	ddtrace "github.com/DataDog/dd-trace-go/opentracing"
 	"github.com/miekg/dns"
 	ot "github.com/opentracing/opentracing-go"
 	zipkin "github.com/openzipkin/zipkin-go-opentracing"
-	"golang.org/x/net/context"
 )
 
 type trace struct {