Implement propagate cache process

[cheme]

Propagate of value must be done manually, some might need to be resend periodically in order to ensure some infos are up to date or a revokation succeed in propagating in the network.
The application is responsible for that.
Yet this look like a common use case enough to give a standard simple implementation.
The idea is to use a cache of Key (not keyval) with there repropagate delay and all such rules...