Implemented WithTimestamp function in gocql.Batch

This function is an addition to DefaultTimestamp function.
Using this function enables defaultTimestamp flag, but instead of
generating new timestamp, it is possible to set it's value explicitly.

It's equal to USING TIMESTAMP x in the statement, but does not break
prepared query optimization.

Similar to #706 PR but for batches.

frame.go

@@ -1414,8 +1414,9 @@ type writeBatchFrame struct {
 	consistency Consistency
 
 	// v3+
-	serialConsistency SerialConsistency
-	defaultTimestamp  bool
+	serialConsistency     SerialConsistency
+	defaultTimestamp      bool
+	defaultTimestampValue int64
 }
 
 func (w *writeBatchFrame) writeFrame(framer *framer, streamID int) error {
@@ -1469,9 +1470,15 @@ func (f *framer) writeBatchFrame(streamID int, w *writeBatchFrame) error {
 		if w.serialConsistency > 0 {
 			f.writeConsistency(Consistency(w.serialConsistency))
 		}
+
 		if w.defaultTimestamp {
-			now := time.Now().UnixNano() / 1000
-			f.writeLong(now)
+			var ts int64
+			if w.defaultTimestampValue != 0 {
+				ts = w.defaultTimestampValue
+			} else {
+				ts = time.Now().UnixNano() / 1000
+			}
+			f.writeLong(ts)
 		}
 	}
 

session.go

@@ -1162,15 +1162,16 @@ func (n *nextIter) fetch() *Iter {
 }
 
 type Batch struct {
-	Type             BatchType
-	Entries          []BatchEntry
-	Cons             Consistency
-	rt               RetryPolicy
-	attempts         int
-	totalLatency     int64
-	serialCons       SerialConsistency
-	defaultTimestamp bool
-	context          context.Context
+	Type                  BatchType
+	Entries               []BatchEntry
+	Cons                  Consistency
+	rt                    RetryPolicy
+	attempts              int
+	totalLatency          int64
+	serialCons            SerialConsistency
+	defaultTimestamp      bool
+	defaultTimestampValue int64
+	context               context.Context
 }
 
 // NewBatch creates a new batch operation without defaults from the cluster
@@ -1263,6 +1264,18 @@ func (b *Batch) DefaultTimestamp(enable bool) *Batch {
 	return b
 }
 
+// WithTimestamp will enable the with default timestamp flag on the query
+// like DefaultTimestamp does. But also allows to define value for timestamp.
+// It works the same way as USING TIMESTAMP in the query itself, but
+// should not break prepared query optimization
+//
+// Only available on protocol >= 3
+func (b *Batch) WithTimestamp(timestamp int64) *Batch {
+	b.DefaultTimestamp(true)
+	b.defaultTimestampValue = timestamp
+	return b
+}
+
 func (b *Batch) attempt(d time.Duration) {
 	b.attempts++
 	b.totalLatency += d.Nanoseconds()