mysql: update mysql driver to latest master

The latest master implements the ColumnType interface.

See https://github.com/go-sql-driver/mysql/issues/495

This removes the custom code in the vendor folder that
did a ColumnType mapping.

vendor/github.com/go-sql-driver/mysql/infile.go

@@ -147,7 +147,8 @@ func (mc *mysqlConn) handleInFileRequest(name string) (err error) {
 	}
 
 	// send content packets
-	if err == nil {
+	// if packetSize == 0, the Reader contains no data
+	if err == nil && packetSize > 0 {
 		data := make([]byte, 4+packetSize)
 		var n int
 		for err == nil {