Pubsub: Implement #expire_item

[lovetox]

Feature:

Let client set a time in seconds when a pubsub item should expire

see XEP-0060

Motivation:

This is nice for user data like User Activity / User Mood / User Tune, or even in the future a Status Message XEP via PEP.

If the user sets his Activity or Mood, the client wants to expire it after some time. Right now we remove the items when going offline. But thats not optimal as there could be other clients online.

Its considerable work to check if an item in a multi client setup with various connection disruptions is still valid or current, a simple "This expires in 24 hours" would be perfect.