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/cache/handler.go

@@ -8,9 +8,10 @@ import (
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/request"
 
+	"context"
+
 	"github.com/miekg/dns"
 	"github.com/prometheus/client_golang/prometheus"
-	"golang.org/x/net/context"
 )
 
 // ServeDNS implements the plugin.Handler interface.