Add expire_after parameter to CachedSession

[m-appel]

Description

The cache of the PeeringDB crawler now expires after six days. Therefore, the weekly dump will always fetch fresh data.

Motivation and Context

Fixes #114 .

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[m-appel]

While I'd argue that it's not a magic number, since the assignment to the days= parameter should make its meaning pretty clear, we can put it in the configuration, maybe as a generic option (i.e., independent of the PeeringDB crawlers, since we might use CachedSession in the future for other crawlers.

How about config['cache']['duration_in_days']? We could also add the directory there:

{
    "cache": {
        "directory": "tmp/",
        "duration_in_days": 6
    }
}

[romain-fontugne]

yes, that sounds good to me. So if we increase the frequency of dumps, we can just modify the configuration file and we don't have to touch the code

[m-appel]

We now read the directory and duration from the config. (I think at some point we should also do something about how we handle the config file :D) I've set the default behavior (i.e., when run without config) to "Do not cache".