Allow scheduling of CF completion in parallel

[fxbonnet]

When a batch loading function returns, we currently complete all CompletableFutures sequentially on the calling thread. Since CompletableFuture.complete(...) also executes any continuations attached via thenApply(...) or thenCompose(...), large batches can significantly delay the return of dispatch().

This PR introduces an option to execute these continuations in parallel instead.

[bbakerman]

This misses the Try path of completion

[andimarek]

@fxbonnet I am not sure about this PR: with the new hook introduced by @bbakerman in #275 you can already offload every completion onto a new thread and hence it will run in parallel if you choose todo so. I have problems seeing how this approach is different/better.

[fxbonnet]

@andimarek I created this PR after my comment on @bbakerman 's PR #275 (comment)
The PR allowed to offload the completions to a separate thread, but in case the batch load contains multiple keys, the completions for each key would still happen sequentially.
For clarity I just rebased this PR onto @bbakerman 's branch. The main change is that instead of passing a single Runnable to scheduleCompletion, we would pass a List<Runnable> (one for each key), giving the option to execute them in parallel.

[fxbonnet]

clearCacheKeys may now be accessed concurrently by multiple thread if we execute the Runnables in parallel.

[bbakerman]

good call - price of side effects outside a thread