Kill req.grace (grace is now a VCL decision, so it's unneeded).
Kill req.keep (it never made any sense in the first place).
Eliminate req->exp, there's just req.d_ttl left now.
Revamp descriptions of $default_(ttl,grace,keep)
Assign obj.keep and obj.grace from defaults on obj creation.

Losse EXP_Grace() and EXP_Keep() functions
Go over the entire struct exp-thing and simplify it a lot.

We now kill objects when t_origin + ttl + grace + keep has elapsed.
This AZ introduced the opposite race of the one it was supposed
to check:  The req might get rescheduled before we get there.
Polish, yet again, specific sizes to make this pass on both
32 and 64 bit architectures.

The good news is that this is necessary because our obj overhead
decreases.