BUG/MEDIUM: mux-h2: always restart reading if data are available

BUG/MEDIUM: mux-h2: always restart reading if data are available

h2c_restart_reading() is used at various place to resume processing of
demux data, but this one refrains from doing so if the mux is already
subscribed for receiving. It just happens that even if some incoming
frame processing is interrupted, the mux is always subscribed for
receiving, so this condition alone is not enough, it must be combined
with the fact that the demux buffer is empty, otherwise some resume
events are lost. This typically happens when we refrain from processing
some incoming data due to missing room in the stream's rxbuf, and want
to resume in h2c_rcv_buf(). It will become even more visible with trailers
since these ones want to have an empty rxbuf before proceeding.

This must be backported to 1.9.