There is no cancellation mechanism in the middle of an HTTP/1
transaction besides closing the session. If a backend closes
the connection before the end of the bereq delivery, it may
also send a 4XX or 5XX response that could be valuable for the
original client.

There are several things to take into consideration: it is
the responsibility of the backend implementation to raise the
bo->send_failed flag, if the backend replied something before
closing we shouldn't trigger a timeout, the connection must not
be recycled, and the object must not be cached.

Fixes #2332
Closes #3686