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 got it. 

Fixed in 4b9761a in PR #186.