BUG/MEDIUM: stconn: Report send activity during mux-to-mux fast-forward

BUG/MEDIUM: stconn: Report send activity during mux-to-mux fast-forward

When data are directly forwarded from a mux to the opposite one, we must not
forget to report send activity when data are successfully sent or report a
blocked send with data are blocked. It is important because otherwise, if
the transfer is quite long, longer than the client or server timeout, an
error may be triggered because the write timeout is reached.

H1, H2 and PT muxes are concerned. To fix the issue, The done_fastword()
callback now returns the amount of data consummed. This way it is possible
to update/reset the FSB data accordingly.

No backport needed.