Bug 3806: Caching responses with Vary header

Bug 3806: Caching responses with Vary header

Various fixes making Vary caching work better.
More work is needed to re-enable shared memory caching of Vary responses.

bag5s r12741: Do not start storing the vary marker object until its key becomes public.
bag5s r12742: Log failed (due to "Vary object loop" or "URL mismatch") hits as TCP_MISSes.
bag5s r12743: Refuse to cache Vary-controlled objects in shared memory (for now).

Based on trunk r13179.