Currently it was possible that PopAll returns 1010 items, then
makes one RPC call with 1000 items, then RPC call 10 items. Meanwhile,
there have been added 500 new items added to the queue.
This change ensures that we pull items from the queue early and
try to make rpc batches as large as possible.
Change-Id: I1a30dde9164c2ff7b90c906a9544593c4f1cf0e9
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
