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/proxy/grpc_test.go

@@ -9,8 +9,9 @@ import (
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/coredns/coredns/request"
 
+	"context"
+
 	"github.com/miekg/dns"
-	"golang.org/x/net/context"
 	"google.golang.org/grpc/grpclog"
 )