Convert large objects to prepared statements

This allows removing semi-obsolete fastpath interface support. See
https://www.postgresql.org/docs/current/libpq-fastpath.html. This also
simplifies introducing context support.

conn.go

@@ -38,7 +38,6 @@ type Conn struct {
 	preparedStatements map[string]*PreparedStatement
 	logger             Logger
 	logLevel           LogLevel
-	fp                 *fastpath
 
 	causeOfDeath error