Write a proposal for how to change the list of private addresses

There are problems if clients and nodes disagree about which addresses are private; if a client requests an address from a node and the node thinks it's private and rejects it, but the address is not blacklisted in the node's exit policy, the client will falsely reject the exit as misbehaving. We need a proposal to handle this in such a way as to let us update the list of private addresses for such tickets as legacy/trac#7971 (moved) without breaking things.