MINOR: mux-h1: rework the h1_timeout_task() function

MINOR: mux-h1: rework the h1_timeout_task() function

Mainly to make it easier to read. First of all, when a H1 connection is
still there, we check if the connection was stolen by another thread or
not. If yes we release the task and leave. Then we check if the task is
expired or not. Only expired tasks are considered. Finally, if a conn-stream
is still attached to the connection (H1C_F_CS_ATTACHED flag set), we
return. Otherwise, the task and the H1 connection are released.