To handle concurrent deletion requests we need to combine them into a
single request.
To implement this we introduces few concurrency ideas:
* Combiner, which takes a node id and a Job and handles combining
multiple requests to a single batch.
* Job, which represents deleting of multiple piece ids with a
notification mechanism to the caller.
* Queue, which provides communication from Combiner to Handler.
It can limit the number of requests per work queue.
* Handler, which takes an active Queue and processes it until it has
consumed all the jobs.
It can provide limits to handling concurrency.
Change-Id: I3299325534abad4bae66969ffa16c6ed95d5574f
