BUG/MEDIUM: stconn: Set fsb date if zero-copy forwarding is blocked during nego

BUG/MEDIUM: stconn: Set fsb date if zero-copy forwarding is blocked during nego

During the zero-copy forwarding, if the consumer side reports it is blocked,
it means it is blocked on send. At the stream-connector level, the event
must be reported to be sure to set/update the fsb date. Otherwise, write
timeouts cannot be properly reported. If this happens when no other timeout
is armed, this freezes the stream.

This patch must be backported to 2.9.

(cherry picked from commit 7cc41514226a2072b538c859cd83dc01de0aeb3f)
Signed-off-by: Willy Tarreau <w@1wt.eu>