MINOR: mux-h1: Rework h1_refresh_timeout to be easier to read

MINOR: mux-h1: Rework h1_refresh_timeout to be easier to read

Conditions to set a timeout on the H1C task have been simplified or at least
changed to rely on H1 connection flags. Now, following rules are used :

 * the shutdown timeout is applied on dead (not alive) or shutted down
   connections.

 * The client/server timeout is applied if there are still some pending
   outgoing data.

 * The client timeout is applied on alive frontend connections with no
   conn-stream. It means on idle or embryionic frontend connections.

 * For all other connections (backend or attached connections), no timeout
   is applied. For frontend or backend attached connections, the timeout is
   handled by the application layer. For idle backend connections, there is
   no timeout.