* 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