Use pgproto3 for startup message

conn.go

@@ -302,27 +302,30 @@ func (c *Conn) connect(config ConnConfig, network, address string, tlsConfig *tl
 		return err
 	}
 
-	msg := newStartupMessage()
+	startupMsg := pgproto3.StartupMessage{
+		ProtocolVersion: pgproto3.ProtocolVersionNumber,
+		Parameters:      make(map[string]string),
+	}
 
 	// Default to disabling TLS renegotiation.
 	//
 	// Go does not support (https://github.com/golang/go/issues/5742)
 	// PostgreSQL recommends disabling (http://www.postgresql.org/docs/9.4/static/runtime-config-connection.html#GUC-SSL-RENEGOTIATION-LIMIT)
 	if tlsConfig != nil {
-		msg.options["ssl_renegotiation_limit"] = "0"
+		startupMsg.Parameters["ssl_renegotiation_limit"] = "0"
 	}
 
 	// Copy default run-time params
 	for k, v := range config.RuntimeParams {
-		msg.options[k] = v
+		startupMsg.Parameters[k] = v
 	}
 
-	msg.options["user"] = c.config.User
+	startupMsg.Parameters["user"] = c.config.User
 	if c.config.Database != "" {
-		msg.options["database"] = c.config.Database
+		startupMsg.Parameters["database"] = c.config.Database
 	}
 
-	if err = c.txStartupMessage(msg); err != nil {
+	if _, err := c.conn.Write(startupMsg.Encode(nil)); err != nil {
 		return err
 	}
 
@@ -1272,11 +1275,6 @@ func (c *Conn) startTLS(tlsConfig *tls.Config) (err error) {
 	return nil
 }
 
-func (c *Conn) txStartupMessage(msg *startupMessage) error {
-	_, err := c.conn.Write(msg.Bytes())
-	return err
-}
-
 func (c *Conn) txPasswordMessage(password string) (err error) {
 	buf := c.wbuf
 	buf = append(buf, 'p')

messages.go

@@ -1,43 +1,15 @@
 package pgx
 
 import (
-	"encoding/binary"
-
 	"github.com/jackc/pgx/pgtype"
 )
 
-const (
-	protocolVersionNumber = 196608 // 3.0
-)
-
 const (
 	copyData = 'd'
 	copyFail = 'f'
 	copyDone = 'c'
 )
 
-type startupMessage struct {
-	options map[string]string
-}
-
-func newStartupMessage() *startupMessage {
-	return &startupMessage{map[string]string{}}
-}
-
-func (s *startupMessage) Bytes() (buf []byte) {
-	buf = make([]byte, 8, 128)
-	binary.BigEndian.PutUint32(buf[4:8], uint32(protocolVersionNumber))
-	for key, value := range s.options {
-		buf = append(buf, key...)
-		buf = append(buf, 0)
-		buf = append(buf, value...)
-		buf = append(buf, 0)
-	}
-	buf = append(buf, ("\000")...)
-	binary.BigEndian.PutUint32(buf[0:4], uint32(len(buf)))
-	return buf
-}
-
 type FieldDescription struct {
 	Name            string
 	Table           pgtype.Oid