MEDIUM: stconn/muxes: Loop on data fast-forwarding to forward at least a buffer

MEDIUM: stconn/muxes: Loop on data fast-forwarding to forward at least a buffer

In the mux-to-mux data forwarding, we now try, as far as possible to send at
least a buffer. Of course, if the consumer side is congested or if nothing
more can be received, we leave. But the idea is to retry to fast-forward
data if less than a buffer was forwarded. It is only performed for buffer
fast-forwarding, not splicing.

The idea behind this patch is to optimise the forwarding, when a first
forward was performed to complete a buffer with some existing data. In this
case, the amount of data forwarded is artificially limited because we are
using a non-empty buffer. But without this limitation, it is highly probable
that a full buffer could have been sent. And indeed, with H2 client, a
significant improvement was observed during our test.

To do so, .done_fastfwd() callback function must be able to deal with
interim forwards. Especially for the H2 mux, to remove H2_SF_NOTIFIED flags
on the H2S on the last call only. Otherwise, the H2 stream can be blocked by
itself because it is in the send_list. IOBUF_FL_INTERIM_FF iobuf flag is
used to notify the consumer it is not the last call. This flag is then
removed on the last call.