Varnish: remove send_timeout=30, rely on the default (600s)

Varnish 3.0.3 introduced idle_send_timeout with a default value of 60,
and increased send_timeout's default to 600, from 60. We had
send_timeout set to 30s, but the codepath where is this is checked is
after idle_send_timeout (SO_SNDTIMEO) fires, so effectively send_timeout
was implicitly set to 60s.

This means that connections were reset after 60s, regardless of their
activity. For this to trigger, 60s had to pass before the TCP send
buffer emptied, which is set to 16MB in our case.

The end-user effect of all this is clients with connections slower than
~270K/s had issues (connection reset) fetching files that needed more
than 60s to finish (i.e. 16MB at 270K/s, 8MB at 135K/s and so on).

This doesn't actually retroactively apply it, so we'll also need a
varnishadm param.set on all Varnish frontend/backends.

Fixes RT #5170 / bug #48353.

Change-Id: I7407613912c49bba583768cb7e27b531e1e95647