store number of requeued workloads
store number of requeued workloads

Summary: Stores an int to keep track of the size of the `requeued_parallelizable` "queue" (stack). This diff makes it so that the overall size of the `WorkloadStream` queue is knowable in constant time, so that in the next diff we can include the queue size in our logs.

Reviewed By: mroch

Differential Revision: D30582683

fbshipit-source-id: a223049e32aec80a9f7f7e596eaf7409267b8e33

facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow