Key delegated to block computing

https://forum.duniter.org/t/idee-cle-deleguee-au-calcul-de-blocs/2698/1

The idea of the delegated key is to be able to delegate the computation of blocks to a subkey, in order to put the main keychain in safety.

This subkey has the ability to revoke the associated member account.

The member may at any time declare a new delegated Key that cancels and replaces the previous delegated Key.

To avoid spam it will be necessary to add a parameter preventing to declare a new delegated key before x seconds.