Consider pulling AsyncLightblueClient into lightblue-client from esbtools/event-handler

[alechenninger]

See esbtools/event-handler#57

Basically, to more efficiently process batches of events, we have an asynchronous client interface with an implementation which lazily performs all requests so they can be batched together in one call. I talked about with @jewzaam about this a few weeks ago and he suggested moving it to the lightblue client library since it is pretty general purpose.

Other implementations could use a thread pool, or whatever.

I don't expect many use cases where this would be essential though since most Java applications are handling requests within a preexisting thread pool. But it is useful for different concurrency models, like batch jobs.