Prepare DatabaseMysql for mysqli

PHP extensions mysql and mysqli have lots in common.
In order to support mysqli, this diff separates common MySQL behavior to abstract class DatabaseMysqlBase and implements the mysql-extension specific in DatabaseMysql.
Outside behavior remains the same.

I've tried to minimize future code duplication by introducing mysql*() methods even if the parent method just unwraps the result.

Bug: 45288
Change-Id: I905d4a4550377bc849a860f0962dad710d9dc71f