Use `tokio::fs::File`

[jdockerty]

After the introduction of tokio as a runtime (#7), there are still a few extra items which would be useful to get through.

Converting from the std::fs::File usage into tokio::fs::File would be useful for the many file I/O operations that are taken in the store. Most notably, I believe this would be largely beneficial for the set operation, as concurrent writes can .await insertion to the file.

This will require a change to the BufReaderWithPos too. It will need to implement AsyncRead, instead of its current Read.