Session::last_active

[Nuhvi]

Add session last active to keep track of old unused sessions.

Biggest challenge in this is to not write to LMDB too often, so we will need a buffer, which will be useful for other features like keeping track of users whose data were never accessed for a while, allowing deleting them or archiving their data and at least stop republishing their keys.

[Nuhvi]

An alternative here is to use timeout / expiration date, cleanup old sessions, and refresh sessions on usuage.