Have a code that persists progress

[dkropachev]

When this library is used by itself it needs to persist progress somewhere.
Currently we don't provide any code that would do that which confuses users.
Let's have two classes that:

1. Persist progress on disk
2. Persist progress on scylla

When we implement them we also need to mention them on README.md alongside with mentioning a problem of having a need to persist the progress.