Persist state on disk

[ajuvercr]

Currently the code is split into two parts: the (fragment) fetcher and the (member) manager, both need state to remember what has been seen before. For this state objects need to exists that can persist their information to disk.

This issue is resolved when

• a state object can persist its data to disk at some location
• hooks into the application runtime are present that fire when the application is about to exit
• a state object can be built from a file on disk that is created by a state object

I think later on, both the fetcher and the manager will need to persist multiple arrays of things, it would be nice if this can already be done with these state objects.

[ajuvercr]

@kkostov Pieter told me you would like to help with this cli application, I think this issue is a great start. It may not be super complex or anything and I know it is below your capabilities, but as long as the main logic is not there yet, it is good to do things without interfering with each other. So if you want, go ahead, if not don't :stuck_out_tongue_closed_eyes: