As @Dridi and myself concluded, the send_timeout had no effect on
backend connections anyway because we never set SO_SNDTIMEO (aka
idle_send_timeout on the client side) on backend connections.

With the next commit, we will fix the send_timeout on the client side and
thus would also enable it for "dripping" writes on the backend side.

To preserve existing behavior for the time being, we explicitly disable
the timeout (actually deadline) on the backend side. There is ongoing
work in progress to rework all of our timeouts for 7.x.

Implementation note: if (VTIM_real() > v1l->deadline) evaluates to false
for v1l->deadline == NaN

Ref #3189