MINOR: mux-h1: Add a flag to disable reads to wait opposite side

MINOR: mux-h1: Add a flag to disable reads to wait opposite side

H1C_F_WAIT_OPPOSITE must be set on the H1 conenction to don't read more data
because we must be sync with the opposite side. This flag replaces the
H1C_F_IN_BUSY flag. Its name is a bit explicit. It is automatically set on
the backend side when the mux is created. It is safe to do so because at
this stage, the request has not yet been sent to the server. This way, in
h1_recv_allowed(), a test on this flag is enough to block the reads instead
of testing the H1 stream state on the backend side.