[BUG] fix random pauses on last segment of a series

[BUG] fix random pauses on last segment of a series

During a direct data transfer from the server to the client, if the
system did not have enough buffers anymore, haproxy would not enable
write polling again if it could write at least one data chunk. Under
normal conditions, this would remain undetected because the remaining
data would be pushed by next data chunks.

However, when this happens on the last chunk of a session, or the last
in a series in an interactive bidirectional TCP transfer, haproxy would
only start sending again when the read timeout was reached on the side
it stopped writing, causing long pauses on some protocols such as SQL.

This bug was reported by an Exceliance customer who generously offered
to help us by sending large amounts of traces and running various tests
on production systems.

It is quite hard to trigger it but it becomes easier with a ping-pong
TCP service which transfers random data sizes, with a modified version
of send() able to send packets smaller than the average transfer size.

A cleaner fix would imply only updating the write timeout when data
transfers are *attempted*, not succeeded, but that requires more
sensible code changes without fixing the result. It is a candidate
for a later patch though.