plugin/dnstap: some cleanup (#1172)

Some cleanup in proxy and dnstap:
* just use time pkg directly and side step the indirection for Epoch
* Use Set in SetQueryEpoch to be more Go like. (Looked like a reader)
* Don't maintain two sets of time, we already track start, so use that.
* Use time.Time and convert when needed
* dedent the toDnstap function and put in a separate file

plugin/proxy/dnstap.go

@@ -0,0 +1,54 @@
+package proxy
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/plugin/dnstap"
+	"github.com/coredns/coredns/request"
+
+	tap "github.com/dnstap/golang-dnstap"
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func toDnstap(ctx context.Context, host string, ex Exchanger, state request.Request, reply *dns.Msg, start time.Time) error {
+	tapper := dnstap.TapperFromContext(ctx)
+	if tapper == nil {
+		return nil
+	}
+
+	// Query
+	b := tapper.TapBuilder()
+	b.TimeSec = uint64(start.Unix())
+	if err := b.HostPort(host); err != nil {
+		return err
+	}
+
+	t := ex.Transport()
+	if t == "" {
+		t = state.Proto()
+	}
+	if t == "tcp" {
+		b.SocketProto = tap.SocketProtocol_TCP
+	} else {
+		b.SocketProto = tap.SocketProtocol_UDP
+	}
+
+	if err := b.Msg(state.Req); err != nil {
+		return err
+	}
+
+	if err := tapper.TapMessage(b.ToOutsideQuery(tap.Message_FORWARDER_QUERY)); err != nil {
+		return err
+	}
+
+	// Response
+	if reply != nil {
+		b.TimeSec = uint64(time.Now().Unix())
+		if err := b.Msg(reply); err != nil {
+			return err
+		}
+		return tapper.TapMessage(b.ToOutsideResponse(tap.Message_FORWARDER_RESPONSE))
+	}
+	return nil
+}