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/dnstap/msg/msg.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"net"
 	"strconv"
-	"time"
 
 	tap "github.com/dnstap/golang-dnstap"
 	"github.com/miekg/dns"
@@ -103,16 +102,6 @@ func (d *Data) Pack(m *dns.Msg) error {
 	return nil
 }
 
-// Epoch returns the epoch time in seconds.
-func Epoch() uint64 {
-	return uint64(time.Now().Unix())
-}
-
-// Epoch sets the dnstap message epoch.
-func (d *Data) Epoch() {
-	d.TimeSec = Epoch()
-}
-
 // ToClientResponse transforms Data into a client response message.
 func (d *Data) ToClientResponse() *tap.Message {
 	t := tap.Message_CLIENT_RESPONSE