ICS3: Add Connection Upgrade Spec (#621)

* connection upgrade progress

* initial writeup

* add notes on init and try access control

* address colin suggestions

* Apply suggestions from code review

Co-authored-by: Christopher Goes <cwgoes@pluranimity.org>

* address chris initial review

* add timeout logic

* crossing hellos logic

* migrations and crossing hello cleanup

* Apply suggestions from code review

Co-authored-by: Marius Poke <marius.poke@posteo.de>

* continue addressing @mpoke suggestions

* clarify handshake pseudocode

* remove unnecessary structs and simplify protocol

* clarify TRY actor reasoning

* TRY timeout clarification

* cleanup pseudocode

* fix mistakes after readthrough

* chan->conn

* Apply suggestions from code review

Co-authored-by: Marius Poke <marius.poke@posteo.de>

* complete fixing minor issues

* reorder utility function and fix timeout conditional bug

* fix restore connection logic

* use 02-client generic verify methods

* address colin comments

* Apply suggestions from code review

Co-authored-by: Marius Poke <marius.poke@posteo.de>

* address rest of reviews

* use connection method

Co-authored-by: Christopher Goes <cwgoes@pluranimity.org>
Co-authored-by: Marius Poke <marius.poke@posteo.de>