Implement liveliness token undeclaration after connectivity lost

[sashacmc]

          Another general comment:

As far as I understood, disconnection is not really managed yet run pico. But theoretically (and in Rust) when connectivity is lost, all remote tokens should be undeclared: an active liveliness subscriber should receive DEL messages for all alive remote tokens, and a liveliness get performed after the disconnection should get no replies.
Not sure if we want to handle that in this PR or another one.

Originally posted by @OlivierHecart in #632 (comment)