* node/approval-voting: Introduce Backend trait and Overlaybackend

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

* node/approval-voting: Add WriteOps for StoredBlockRange and BlocksAtHeight

* node/approval-voting: Add load_all_blocks to overlay

* node/approval-voting: Get all module tests to pass.

This commit modifies all tests to ensure tests are passing.

* node/approval-voting: Address oversights in the previous commit

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

* node/approval-voting: Move from TestDB to dyn KeyValueDB

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

* node/approval-voting: Address feedback

* node/approval-voting: Add license to ops.rs

* node/approval-voting: Address second-pass feedback

* Add TODO

* node/approval-voting: Bump spec_version

* node/approval-voting: Address final comments.