BUG/MEDIUM: stconn: Move data from <kip> to <kop> during zero-copy forwarding

BUG/MEDIUM: stconn: Move data from <kip> to <kop> during zero-copy forwarding

The <kip> of producer was not forwarded to <kop> of consumer when zero-copy
data forwarding was tried. Because of the issue, the chunking of emitted H1
messages could be invalid.

To fix the bug, sc_ep_fwd_kip() must be called at this stage.

This fix is related to the previous one (529a8dbfb "BUG/MEDIUM: mux-h1: Take
care to update <kop> value during zero-copy forwarding"). Both are required
to fully fix the issue #3230.

This patch must be backported to 3.3.