Restructure sending messages

Use an internal buffer in pgproto3.Frontend and pgproto3.Backend instead
of directly writing to the underlying net.Conn. This will allow tracing
messages as well as simplify pipeline mode.

pgconn/errors.go

@@ -178,23 +178,6 @@ func newContextAlreadyDoneError(ctx context.Context) (err error) {
 	return &errTimeout{&contextAlreadyDoneError{err: ctx.Err()}}
 }
 
-type writeError struct {
-	err         error
-	safeToRetry bool
-}
-
-func (e *writeError) Error() string {
-	return fmt.Sprintf("write failed: %s", e.err.Error())
-}
-
-func (e *writeError) SafeToRetry() bool {
-	return e.safeToRetry
-}
-
-func (e *writeError) Unwrap() error {
-	return e.err
-}
-
 func redactPW(connString string) string {
 	if strings.HasPrefix(connString, "postgres://") || strings.HasPrefix(connString, "postgresql://") {
 		if u, err := url.Parse(connString); err == nil {