Re: Make HeapTupleSatisfiesMVCC more concurrent

I wrote:
> Just thinking about this ... I wonder why we need to call
> TransactionIdIsInProgress() at all rather than believing the answer from
> the snapshot?  Under what circumstances could TransactionIdIsInProgress()
> return true where XidInMVCCSnapshot() had not?

I experimented with the attached patch, which replaces
HeapTupleSatisfiesMVCC's calls of TransactionIdIsInProgress with
XidInMVCCSnapshot, and then as a cross-check has all the "return false"
exits from XidInMVCCSnapshot assert !TransactionIdIsInProgress().
The asserts did not fire in the standard regression tests nor in a
pgbench run, which is surely not proof of anything but it suggests
that I'm not totally nuts.

I wouldn't commit the changes in XidInMVCCSnapshot for real, but
otherwise this is a possibly committable patch.