Re: Make HeapTupleSatisfiesMVCC more concurrent

I don't mean to dismiss the potential for further optimization inside
XidInMVCCSnapshot (for instance, the one-XID-cache idea sounds promising);
but I think that's material for further research and a separate patch.

It's not clear to me if anyone wanted to do further review/testing of
this patch, or if I should go ahead and push it (after fixing whatever
comments need to be fixed).