MINOR: mux-h2: do not emit GOAWAY on idle ping expiration

MINOR: mux-h2: do not emit GOAWAY on idle ping expiration

If idle-ping is activated and h2c task is expired due to missing PING
ACK, consider that the peer is away and the connection can be closed
immediately. GOAWAY emission is thus skipped.

A new test is necessary in h2c_update_timeout() when PING ACK is
currently expected, but the next timer expiration selected is not
idle-ping. This may happen if http-keep-alive/http-request timers are
selected first. In this case, H2_CF_IDL_PING_SENT flag is resetted. This
is necessary to not prevent GOAWAY emission on expiration.