Abstract SnapshotWorker and durability::Local

[Shubham8287]

Description of Changes

• Introduce SnapshotRepo as the snapshot backend trait and decouple SnapshotWorker from the
  filesystem-backed implementation.

  • Dynamic dispatch-ing is not needed but done to avoid propagating generics all over the place.
  • Snapshotting is also not on the hot path, and durability also follows the same dyn pattern.

• Make commitlog durability::Local generic over Repo, so the same durability implementation can be
  reused with in-memory and fault-injected repositories.

• Add a few small API exposure changes under cfg(test) for DST integration.

API and ABI breaking changes

NA.

Expected complexity level and risk

2

Testing

Existing tests should be enough.