ADODB_pdo_mysql: add functionality from ADODB_mysql

* SQLDate method for generating date formatting strings
* SelectDB method for changing selected database
* $metaTablesSQL string that includes table vs view detection

Makes pdo_mysql behave more similarly to mysql and mysqli. Swapping underlying driver becomes easier.

Sidenote: for general pull request etiquette, if I have three other distinct commits, is there a preference for one pull request vs three separate ones? Should I create an issue (or issues) before sending a pull request?