Implement pending storage persistence

[appaquet]

Right now, pending store is kept in memory. In order to increase resilience of it in case of crash, we need persist it.

The ideal persistence would be a WAL since the pending store is continuously being appended and cleaned up. We could directly mmap the operations data from it instead of storing them in memory.

Operations would still be in memory, but not their frame data.

TODO:

• [ ] Find a WAL crate that we can reference bytes from
• [ ] Think of a way to cleanup old segments of log