Create an ADR to get rid of the TVar in the Direct module

[ch1bo]

Our goal is to simplify the Hydra.Chain.Direct module such that it is not having state on it's own in a TVar, but we can integrate it's state into the general state of the Hydra.Node to simplify rollbacks & persistence later.

This ADR shall be the basis for next steps into this direction and ultimately to solve #184

Discussion

Make observation stateless:

• Problem is that we need to keep the UTxO around in order to understand the tx
• Observation is stateful in the sense it needs the "state" of the ledger, which is either the list of txs or the UTxO
• What we have now is good enough, we only change the interface between Node and Chain to add a "callback" as continuation?
• A step in the "right" direction would be to move state into a "middleware" layer and make the Tx stateless -> we already did that?

Also relevant: #184 (comment)

[ch1bo]

Written down my ideas before heading off into some vacation. This is still a VERY MUCH WIP draft of things: https://github.com/input-output-hk/hydra-poc/tree/ch1bo/adr-18-single-state