improve checking of user credentials in petascope

[Dimitar Misev]

Instead of executing a full SELECT 1 query, to check credentials petascope just needs to do

RasImplementation impl = new RasImplementation(ConfigManager.RASDAMAN_URL);
impl.setUserIdentification(username, password);

If credentials are invalid it will throw an exception similar to org.odmg.ODMGRuntimeException: The password provided by the client for user 'X' is invalid., otherwise credentials are valid.

Another improvement is to cache the RasImplementation impl objects for each username/password combination, because it's not so cheap to reauthenticate for each query.