BUG/MEDIUM: mux-h1: Stop sending vi fast-forward for unexpected states

BUG/MEDIUM: mux-h1: Stop sending vi fast-forward for unexpected states

If a producer tries to send data via the fast-forward mechanism while the
message is in an unexpected state from the consumer point of view, the
fast-forward is now disabled. Concretely, we now take care that the message
is in its data/tunnel stage to proceed in h1_nego_ff().

By disabling fast-forward in that case, we will automatically fall back on
the regular sending path and be able to handle the error in h1_snd_buf().

This patch should be backported as far as 3.0