Fix some more phpdoc issues with returned values. Add a getLastError() method to retrieve information about the last error occurred.

Fix build: given that username and password are optional for some PDO drivers, do not enforce them to be set in the configuration. Use null as a default value for them.

Generate the instance ID from the configuration itself instead of the spl_object_id of the configuration class

Created a central database class and updated the PDO MetaDataStorangeHandler to use it instead of its own database classes