PERF: don't ask for new posts while loading new posts (PR #10937)

Previous to this change we had no protection to ensure we wait on a request for more posts prior starting another request.

In outlier cases if 10 people post at the same time on a topic a flood of requests could start.

To improve this situation we now ensure that we are done asking for new posts prior to asking for the next batch.

Also addresses some style issues raised previously and moves init to top of class.


I think @jjaffeux brought this up but we really need to store a handle to any timeouts (or laters) and cancel them accordingly.