BUG/MEDIUM: stconn: Fix conditions to know an applet can get data from stream

BUG/MEDIUM: stconn: Fix conditions to know an applet can get data from stream

sc_is_send_allowed() function is used to know if an applet is able to
receive data from the stream. But this function was designed for applets
using the channels buffer. It is not adapted to applets using their own
buffers.

when the SE_FL_WAIT_DATA flag is set, it means the applet is waiting for
more data and should not be woken up without new data. For applets using
channels buffer, just testing the flag is enough because process_stream()
will remove if when more data will be available. For applets using their own
buffers, it is more complicated. Some data may be blocked in the output
channel buffer. In that case, and when the applet input buffer can receive
daa, the applet can be woken up.

This patch must be backported as far as 3.0 after a period of observation.

(cherry picked from commit 41a40680ce1c513d49a40d5e9f3c80a8a70a369f)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit ed3273f771c778527a19f85bd646b7c4a21ce0d9)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 22210bb66f77d87cd8004105e029ef3f893ed3d7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>