ignore mysql-specific comment statements

[slanning]

This is related to #3520

Ignore /*! mysql-specific */ and /*!50708 mysql-version-specific */ comments.
I'm not sure if it's a good idea. Instead of stripping leading comments,
in case it's /*! ... */ we now don't strip it, and consider it a new StmtComment statement type.
(Not sure if that should be added to ast.go . I didn't.)

I assumed this kind of comment was the only thing in the query,
so if there's something like "/*! ... */ select ..." it wouldn't work.
The handleComment in executor.go basically does nothing, returning &sqltypes.Result{}

[derekperkins]

I think this will help with migrations. I ran into this yesterday when trying to import a default mysqldump, but vtgate choked on all these standard statements.

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;

[slanning]

Hopefully. :) I'm not sure if what I did makes it not send the SQL to any vttablets, or if it goes through to all of them by default somehow. I think it doesn't pass it through... So I worry that in that mysqldump case, if the SET commands aren't passed through, it might cause data corruption (wrong time zone, character set).

[sougou]

LGTM