Re: Single pass vacuum - take 1

I think we are better off doing only equality comparisons and dodging
this problem altogether.

Just-plain-dead line pointers would have lp_off = 0.  Dead-vacuumed
line pointers would have lp_off != 0.  The first vacuum would use
lp_off = 1, the next one lp_off = 2, etc.

Actually, come to think of it, we could fit a 30-bit counter into the
line pointer.  There are 15 unused bits in lp_off and 15 unused bits
in lp_len.

If we use a counter that is large enough that we don't have to worry
about wrap-around, I guess that's OK, though it seems a little weird
to think about having different backends running with different ideas
about the correct counter value.