Feature request: Selfdestroying / temporary updates

# What
Allow users and site owner to specify how long a `.json` file should be retained. After such amount of time, an automatic deletion should be performed by each ZN client.

# Use cases
In ZeroMail send each mail in a separate file and globally set them to be canceled after e.g. 1 month. This allows the other party to receive the email, and with the archival plugin to have it permanently, while at the same time not cluttering the global hard disk space of everyone.

This could be the scaling solution for all sites where users put some data just to rely on their continuous presence online, but the data is only useful for some of the other users or for a Limited amount of time (for example a user run news site or a board Like zeroboard or for temporary notes on nullpaste)

# Possible problems
There should be a clock that is agreed upon or one can simply let each client use it's own (in ZN code there is already some points where it is assumed that the client clocks are not too far from each other, i.e. Within one day)