[MEDIUM] stream_sock: implement tcp-cork for use during shutdowns on Linux

[MEDIUM] stream_sock: implement tcp-cork for use during shutdowns on Linux

Setting TCP_CORK on a socket before sending the last segment enables
automatic merging of this segment with the FIN from the shutdown()
call. Playing with TCP_CORK is not easy though as we have to track
the status of the TCP_NODELAY flag since both are mutually exclusive.
Doing so saves one more packet per session and offers about 5% more
performance.

There is no reason not to do it, so there is no associated option.