BUG/MEDIUM: stconn: Properly forward kip to the opposite SE descriptor

BUG/MEDIUM: stconn: Properly forward kip to the opposite SE descriptor

By refactoring the HTX to remove the extra field, a bug was introduced in
the stream-connector part. The <kip> (known input payload) value of a sedesc
was moved to <kop> (knwon output payload) using the same sedesc. Of course,
this is totally wrong. <kip> value of a sedesc must be forwarded to the
opposite side.

In addition, the operation is performed in sc_conn_send(). In this function,
we manipulate the stream-connectors. So se_fwd_kip() function was changed to
use the stream-connectors directely.

Now, the function sc_ep_fwd_kip() is now called with the both
stream-connectors to properly forward <kip> from on side to the opposite
side.

The bug is 3.3-specific. No backport needed.