remove old cruft
Give the busyobj a copy of req->digest, and move adding saintmode entries
to use busyobj.
Give the busyobj a copy (+ref) of the req's vcl and director.
Introduce the bereq.backend* variables.

NB: bereq.backend.healthy always fails right now.
Pass the digest to the backend->healthy() check, rather than the req*
Make VDI_Healthy() req/busyobj agnostic.
Start sorting VCL variables into "client", "backend" or "both".

Fix some test-cases to match, generally s/req.*/bereq.*/
Start moving the backend->getfd() stuff from req to busyobj
Push the req->busyobj change all the way up through getfd.