[BISERVER-11415] Refresh connection before retrieving Meta

DatabaseConnection objects passed to public MultitableDatasourceService methods may be de-serialized from the client.
DTOs from the client should never be used to create DatabaseMeta and may be missing extra parameters and attributes.
Instead, resolve the connection by name through ConnectionService before use. All public methods of MultitableDatasourceService should use getDatabaseMeta to guarantee accurate connection info.

Affected Methods:
  * getDatabaseTables
  * retrieveSchemas
  * getTableFields
  * serializeJoins

http://jira.pentaho.com/browse/BISERVER-11415