Allow scheduling of CF completion

[bbakerman]

The completion of CF promises can now be scheduled on another thread

A step in the batch loading process is the completion of the futures that have been promised earlier via
Dataloader.load(). This can also be scheduled on another thread if need be, allowing the dispatch call
to return quicker.

You might want to schedule the completion of values on another thread if there is following work t do such as :

var cfStage1 = dataLoader.load(key);
var cfStage2 = cfStage1.thenApply(v -> doSomethingSlow(v));
//...
var dispatchCF = dataLoader.dispatch();

In the above example the .doSomethingSlow(v) call will happen inside the completion
of the cfStage1 code path since CompletableFuture by design eagerly runs dependent chained methods.

Perhaps you want this completion to happen more asynchronously so that the .dispatch() returns more
quickly and is not bound to the .doSomethingSlow(v) call.

By default, the dispatch completion is done on the current thread in a synchronous manner, which will include
any extra CompletableFuture chained methods.

This is an example of running the completion step in an asynchronous manner :

@Override
public <K> CompletionStage<?> scheduleCompletion(Runnable completeValuesRunnable, List<K> keys, BatchLoaderEnvironment environment) {
    return CompletableFuture.runAsync(completeValuesRunnable);
}

[bbakerman]

We now create the BatchLoaderEnvironment earlier and send it down into the other methods earlier

[bbakerman]

The above is now run in the runnable and hence can be scheduled onto some other thread

[fxbonnet]

This moved the CF completions to another thread but the for loop still invokes them sequentially which is a problem if there are many keys in the batch. Could we schedule the completion for each key in a separate thread instead?

[bbakerman]

So are you saying we pass in

 public CompleableFuture<Void> completeThese(List<CompletableFuture> cfs, List<Object> values)

And the the caller could sequentially run them as today OR choose to run them in parralel themselves say?

[bbakerman]

diff is whacky. This has not changed

[bbakerman]

Called in two places - common method

[bbakerman]

never had tests

[fxbonnet]

Isn't this the same as CompletableFuture.completedFuture(v)?

[bbakerman]

Yes - but here for "completeness" ;)

[fxbonnet]

This moved the CF completions to another thread but the for loop still invokes them sequentially which is a problem if there are many keys in the batch. Could we schedule the completion for each key in a separate thread instead?