On the client side, we impose a total timeout, yet on the client side we
use between_bytes_timeout and do not care about the total (<- we might
want to reconsider this).

Yet HTC_RxStuff only implemented a total timeout, for which we
effectively used first_byte_timeout + between_bytes_timeout. Yet if
first_byte_timeout was not used up, the effective timeout between bytes
could be substantially longer than between_bytes_timeout (initially
analyzed by @daghf).

We now add a duration argument td to HTC_RxStuff which will be used in
addition to (or instead of) the existing total timeout tn. Either td or
tn must be given.

Testcase originally by @fgsch, slightly modified to avoid an assertion
failure in vtc_server due to the connection being closed by varnish.

Fixes #2395