TOAST needs to do at least minimal time-qual checking in order not to

mess up after an aborted VACUUM FULL, per today's pghackers discussion.
Add a suitable HeapTupleSatisfiesToast routine.  Remove useless special-
case test in HeapTupleSatisfiesVisibility macro for xmax =
BootstrapTransactionId; perhaps that was needed at one time, but it's
a waste of cycles now, not to mention actively wrong for SnapshotAny.
Along the way, add some much-needed comments to tqual.c, and simplify
toast_fetch_datum, which no longer needs to assume it may see chunks
out-of-order.