MINOR: conn-stream: Force mux to wait for read events if abortonclose is set

MINOR: conn-stream: Force mux to wait for read events if abortonclose is set

When the abortonclose option is enabled, to be sure to be immediately
notified when a shutdown is received from the client, the frontend
conn-stream must be sure the mux will wait for read events. To do so, the
CO_RFL_KEEP_RECV flag is set when mux->rcv_buf() is called. This new flag
instructs the mux to wait for read events, regardless its internal state.

This patch is required to fix abortonclose option for H1 client connections.