[change] Clean up stale sessions improvements

[nemesifier]

The following:

https://github.com/openwisp/openwisp-radius/blob/e61d2457a1c043dc56dce82f6de5bc6f9e5b7ced/openwisp_radius/management/commands/base/cleanup_stale_radacct.py#L18-L25

Should become a model method. Then we should edit this so that we filter either:

• sessions which have a last update time older than the specified amount of time
• sessions which do not have any last update time set and have the start time older than the specified amount of time (kind of as it is now but with the difference that we take this only if update time is not set)