* node/dispute-coordinator: Modify db to return SubsystemResult.
In preparation of moving to the overlayed backend pattern, this commit
moves the db to return SubsystemResult values.
* node/dispute-coordinator: Add the Backend and OverlayedBackend.
This commit adds the backend and overlayed backend structs to the
dispute-coordinator subsystem.
* node/dispute-coordinator: Implement backend and overlayed-backend.
This commit finalizes the move from the previous transactional model
to the common overlay pattern in subsystem persistency. This can be
observed in the ApprovalVoting and ChainSelection subsystems.
* Add module docs + license
* Touchup merge