Last preparations for replacing ProtoMicroPaymentChannel with MicroPaymentChannel

**Description:** last preparations for replacing ProtoMicroPaymentChannel with MicroPaymentChannel
**Testing:** tests pass
**Coverage:** about the same
If we want to make that distinction I will prefer to have derived classes from `ImmutableTransaction` that can also check some invariants. Then we can have `Both[Deposit]` and `Both[Refund]`. 

I think it is more useful.