Deal with any remaining request body in cnt_synth

Call HTTP1_DiscardReqBody() in cnt_synth before delivery to deal with
any lingering request body.

Fixes: #1577
Set default ttl (according to rfc2616 rules) to max-age, do not use Age twice

With 160927b690dd076702601b7407eb71bd423c62dd t_origin got corrected
by Age, so we already took the age the object had at fetch time into
consideration for all other calculations.

Fixes #1578