feat: add mock fs for wal

[bsbds]

The mock filesystem implementation is used for simulation purposes. For example, we can discard unsynced data during a simulation test using this implementation.

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)