Add cache feature for εxodus trackers database

[pnu-s]

Currently, exodus-standalone downloads εxodus trackers database for each run.

The database being fairly stable (changing only from time to time), the script should store it locally so that it is not downloaded every time (less load on εxodus servers, faster execution, etc.)

What the script could do :

• if no data locally, download it and store it
• if some data is available, check its timestamp and only refresh it if older than X
• if an option such as --refresh-cache is provided, refresh the case anyway