OPTIM: stconn: Don't pretend mux have more data to deliver on EOI/EOS/ERROR

OPTIM: stconn: Don't pretend mux have more data to deliver on EOI/EOS/ERROR

Doing some benchs on the 3.0, we encountered a small loss on requests/sec on
small objects compared to the 2.8 . After bisecting the issue, it appeared
that this was introduced when the mux-to-mux zero-copy data forwarding was
implemented in 2.9-dev8. Extra subscribes on receives at the end of the
message were responsible of the loss.

A basic configuration, sending H2 requests to a H1 server returning
responses without payload is enough to observe the issue. With the following
command, we can observe a huge increase of epoll_ctl calls on 2.9/3.x:

  h2load -c 100 -m 10 -n 100000 http://...

On 2.8 we have around 3200 calls to epoll_ctl against more than 20k on 3.1.

The fix seems obvious. After a receive, there is no reason to state a mux
have more data to deliver if EOI/EOS/ERROR flag was set on the
stream-endpoint descriptor. With this change, extra calls to epoll_ctl
disappear. However it is a sensitive part so it is important to keep an eye
on it and to not backport it.

Thanks to Willy and Emeric to have spot the issue.