Add pruning and WAL capability for SS store

[yzang2019]

Describe your changes and provide context

This PR include several changes:

1. We add a new config for SS store: DedicatedChangelog (Means that we should use a dedicated WAL file)
2. We add a new interface for SS store: ApplyChangesetAsync (Used for supporting async writes)
3. We make pruning part of SS and automatically start when we initialize the DB
4. We add pruning logic for changelog as well
5. We modify recovery logic to backward search the starting position one by one
6. If DedicatedChangelog = true, we will perform a recovery first during initialization

Testing performed to validate your change

Added unit test