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

pb/dns.pb.go

@@ -18,7 +18,8 @@ import fmt "fmt"
 import math "math"
 
 import (
-	context "golang.org/x/net/context"
+	context "context"
+
 	grpc "google.golang.org/grpc"
 )