Implemented WithTimestamp function in gocql.Query

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.

conn.go

@@ -715,6 +715,7 @@ func (c *Conn) executeQuery(qry *Query) *Iter {
 	// frame checks that it is not 0
 	params.serialConsistency = qry.serialCons
 	params.defaultTimestamp = qry.defaultTimestamp
+	params.defaultTimestampValue = qry.defaultTimestampValue
 
 	if len(qry.pageState) > 0 {
 		params.pagingState = qry.pageState

frame.go

@@ -1229,7 +1229,8 @@ type queryParams struct {
 	pagingState       []byte
 	serialConsistency SerialConsistency
 	// v3+
-	defaultTimestamp bool
+	defaultTimestamp      bool
+	defaultTimestampValue int64
 }
 
 func (q queryParams) String() string {
@@ -1301,7 +1302,12 @@ func (f *framer) writeQueryParams(opts *queryParams) {
 
 	if f.proto > protoVersion2 && opts.defaultTimestamp {
 		// timestamp in microseconds
-		ts := time.Now().UnixNano() / 1000
+		var ts int64
+		if opts.defaultTimestampValue != 0 {
+			ts = opts.defaultTimestampValue
+		} else {
+			ts = time.Now().UnixNano() / 1000
+		}
 		f.writeLong(ts)
 	}
 }

session.go

@@ -569,23 +569,24 @@ func (s *Session) connect(addr string, errorHandler ConnErrorHandler, host *Host
 
 // Query represents a CQL statement that can be executed.
 type Query struct {
-	stmt                string
-	values              []interface{}
-	cons                Consistency
-	pageSize            int
-	routingKey          []byte
-	routingKeyBuffer    []byte
-	pageState           []byte
-	prefetch            float64
-	trace               Tracer
-	session             *Session
-	rt                  RetryPolicy
-	binding             func(q *QueryInfo) ([]interface{}, error)
-	attempts            int
-	totalLatency        int64
-	serialCons          SerialConsistency
-	defaultTimestamp    bool
-	disableSkipMetadata bool
+	stmt                  string
+	values                []interface{}
+	cons                  Consistency
+	pageSize              int
+	routingKey            []byte
+	routingKeyBuffer      []byte
+	pageState             []byte
+	prefetch              float64
+	trace                 Tracer
+	session               *Session
+	rt                    RetryPolicy
+	binding               func(q *QueryInfo) ([]interface{}, error)
+	attempts              int
+	totalLatency          int64
+	serialCons            SerialConsistency
+	defaultTimestamp      bool
+	defaultTimestampValue int64
+	disableSkipMetadata   bool
 
 	disableAutoPage bool
 }
@@ -649,6 +650,18 @@ func (q *Query) DefaultTimestamp(enable bool) *Query {
 	return q
 }
 
+// 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 (q *Query) WithTimestamp(timestamp int64) *Query {
+	q.DefaultTimestamp(true)
+	q.defaultTimestampValue = timestamp
+	return q
+}
+
 // RoutingKey sets the routing key to use when a token aware connection
 // pool is used to optimize the routing of this query.
 func (q *Query) RoutingKey(routingKey []byte) *Query {