Fixes for the remainder of lib/SimpleSAML

The use of count function is wrong and unnecessary. If the array is
empty the foreach loop will not run.

Bugfix. While SimpleSAML\Database::query() returns a PDOStatement, SimpleSAML\Database::exec() doesn't. Make both return an integer with the amount of rows affected. Add a return value to SimpleSAML_Metadata_MetadataStorageHandlerPdo::initDatabase().

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