MEDIUM: mux-h1: Add ST_READY state for the H1 connections

MEDIUM: mux-h1: Add ST_READY state for the H1 connections

An alive H1 connection may be in one of these 3 states :

  * ST_IDLE : not active and is waiting to be reused (no h1s and no cs)
  * ST_EMBRYONIC : active with a h1s but without any cs
  * ST_ATTACHED : active with a h1s and a cs

ST_IDLE and ST_ATTACHED are possible for frontend and backend
connection. ST_EMBRYONIC is only possible on the client side, when we are
waiting for the request headers. The last one is the expected state for an
active connection processing data. These states are mutually exclusives.

Now, there is a new state, ST_READY. It may only be set if ST_ATTACHED is
also set and when the CS is considered as fully active. For now, ST_READY is
set in the same time of ST_ATTACHED. But it will be used to fix TCP to H1
upgrades. Idea is to have an H1 connection in ST_ATTACHED state but not
ST_READY yet and have more or less the same behavior than an H1 connection
in ST_EMBRYONIC state. And when the upgrade is fully achieved, the ST_READY
state may be set and the data layer may be notified accordingly.

So for now, this patch should not change anything. TCP to H1 upgrades are
still buggy. But it is mandatory to make it work properly.