Avoid allocation for WriteBuf on query

conn.go

@@ -47,6 +47,7 @@ type Conn struct {
 	lastActivityTime   time.Time     // the last time the connection was used
 	reader             *bufio.Reader // buffered reader to improve read performance
 	wbuf               [1024]byte
+	writeBuf           WriteBuf
 	Pid                int32             // backend pid
 	SecretKey          int32             // key to use to send a cancel query message to the server
 	RuntimeParams      map[string]string // parameters that have been reported by the server

messages.go

@@ -91,7 +91,8 @@ func (self PgError) Error() string {
 
 func newWriteBuf(c *Conn, t byte) *WriteBuf {
 	buf := append(c.wbuf[0:0], t, 0, 0, 0, 0)
-	return &WriteBuf{buf: buf, sizeIdx: 1, conn: c}
+	c.writeBuf = WriteBuf{buf: buf, sizeIdx: 1, conn: c}
+	return &c.writeBuf
 }
 
 // WrifeBuf is used build messages to send to the PostgreSQL server. It is used