MINOR: mux-h2: handle idle-ping on conn reverse

MINOR: mux-h2: handle idle-ping on conn reverse

This commit extends MUX H2 connection reversal step to properly take
into account the new idle-ping feature. It first ensures that h2c task
is properly instantiated/freed depending now on both timers and
idle-ping configuration. Also, h2c_update_timeout() is now called
instead of manually requeuing the task, which ensures the proper timer
value is selected depending on the new connection side.