This ticket adds the dispatcher: the loop that pulls batches from the existing oplog batcher and routes their ops onto the worker queues.

This ticket should:

• Introduce the run loop in the POA that performs dispatching of ops. This loop should pull batches off the oplog batcher, classify them, and then route them to workers.

We should also ensure that fast-count in-memory counts are updated. Note that the dispatcher does not have to handle the drain + inline case yet.