Fix reactive data loader hanging on backpressured publishers (#273)

[zenios]

The reactive subscribers (AbstractBatchSubscriber and friends) requested keys.size() from the upstream publisher exactly once in onSubscribe and then waited for onComplete to finalise the batch.

A reactive publisher only emits while it has outstanding demand. So a publisher that emits values lazily as more demand arrives - for example one that emits an entry not matching any requested key before the entry that does - consumes that single window of demand and then blocks. The matching value is never requested, the per-key future never completes, and the data loader hangs forever.

This change makes the subscribers manage demand properly:

• track the outstanding demand and re-request another window whenever it drains to zero, repeating until the publisher completes or errors;
• once a result has been received for every key there is nothing left to wait for, so cancel the upstream subscription and complete ourselves rather than blocking on a publisher that may never call onComplete.

Adds ReactiveBackpressureTest reproducing the hang for both the list and mapped publisher data loaders; both tests time out against the old code and pass with the fix.

[bbakerman]

Thanks very much for this. Demand management is an art and often gotten wrong.

I also like the idea we can ignore keys we didn't ask for. Probably not likely but possible and hence handling it is a good idea