Refine and test exchange model

**Description:** refine and test the exchange model up to the handshake part. Closer to a complete implementation but we should stop and merge at some point so stuff marked as TODO and MicroPaymentChannel stuff is out of scope.
**Testing:** tests pass!
**Coverage:** about 80% for the `impl` package.
**Reviewer:** @ximo
Ok, I see. Your point is that `createMicroPaymentChannel()` requires a validation of the deposit transaction that includes a confidence type of `BUILDING`. 

Working on it.