Docs: Add proto freeze todo list
After each protocol activation, old protocols are kept frozen in the codebase. However large part of the code developed for an old protocol can now be removed to avoid accumulating dead code in the repository. This code is composed by all daemons, tests associated to old protocols, and RW commands available to the client.
This MR adds a todo list to follow when freezing an old protocol.
This is a spinoff from the MR : !3178 (closed)
Edited by pietro