MINOR: mux-h2: refactor idle timeout calculation

MINOR: mux-h2: refactor idle timeout calculation

Reorganize code for timeout calculation in case the connection is idle.
The objective is to better reflect the relations between each timeouts
as follow :

* if GOAWAY already emitted, use shut-timeout, or if unset fallback to
  client/server one. However, an already set timeout is never erased.

* else, for frontend connection, http-request or keep-alive timeout is
  applied depending on the current demux state. If the selected value is
  unset, fallback to client timeout

* for backend connection, no timeout is set to perform http-reuse

This commit is pure refactoring, so no functional change should occur.