This commit introduces a Backend trait and attempts to move away
from the Action model via an OverlayBackend as in the ChainSelection
subsystem.

…eight

This commit modifies all tests to ensure tests are passing.

This commit addresses some oversights in the prior commit.

1. Inner errors in backend.write were swallowed
2. One-off write functions removed to avoid useless abstraction
3. Touch-ups in general

This commit removes the TestDB from tests.rs and replaces it with
an in-memory kvdb.

…rlay-db