[Java] Refactoring the ledger adaptor

Hey guys,

I've been looking at the ledger adaptor interface and thinking about a few
topics we've discussed lately:

1. Using Futures instead of response events
2. The conceptual move away from "ledgers" toward "payment channels"
3. The move in the JS implementation toward a messaging pattern

I also did some research on Message Oriented Middleware and standards for
message based interop in Java (which basically means JMS).

I am working on some proposed changes which I'll put in a branch and share.

My plan is to make it obvious how you'd implement a connection to a
ledger/channel using standard middle-ware like a JMS provider. (Down the
line we could even ship a JMS provider although it seems most middleware
still only supports JMS 1.1 despite 2.0 being 3 years old).

I also want to expriment with running a ledger inside a container and
connecting to it without serialization (i.e. have the client run inside the
same app container).

Any thoughts?

Adrian