Add Classifier ID validation; Separate thread for Gate poll

Classifier priority of the gates is now 64+Classifier ID. In order for that to work we should probably do a Classifier ID validation: basically since only four classifiers are supported we should only accept 1,2,3 or 4 as a classifier ID.

When we do an RPC gate poll for all the gates of an App, the plugin now uses now a separate thread. This also shortens somewhat the qosPollGates method which I think was a concern. For a large CMTS, there can be hundreds of gates. API request needs to happen quickly, synchronously and then the poll operations can happen in the background.