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

core/dnsserver/server_grpc.go

@@ -1,6 +1,7 @@
 package dnsserver
 
 import (
+	"context"
 	"crypto/tls"
 	"errors"
 	"fmt"
@@ -9,7 +10,6 @@ import (
 	"github.com/grpc-ecosystem/grpc-opentracing/go/otgrpc"
 	"github.com/miekg/dns"
 	"github.com/opentracing/opentracing-go"
-	"golang.org/x/net/context"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/peer"