Improve performance of get_actual_variable_range with recently-dead tuples.

In commit fccebe421, we hacked get_actual_variable_range() to scan the
index with SnapshotDirty, so that if there are many uncommitted tuples
at the end of the index range, it wouldn't laboriously scan through all
of them looking for a live value to return.  However, that didn't fix it
for the case of many recently-dead tuples at the end of the index;
SnapshotDirty recognizes those as committed dead and so we're back to
the same problem.

To improve the situation, invent a "SnapshotNonVacuumable" snapshot type
and use that instead.  The reason this helps is that, if the snapshot
rejects a given index entry, we know that the indexscan will mark that
index entry as killed.  This means the next get_actual_variable_range()
scan will proceed past that entry without visiting the heap, making the
scan a lot faster.  We may end up accepting a recently-dead tuple as
being the estimated extremal value, but that doesn't seem much worse than
the compromise we made before to accept not-yet-committed extremal values.

The cost of the scan is still proportional to the number of dead index
entries at the end of the range, so in the interval after a mass delete
but before VACUUM's cleaned up the mess, it's still possible for
get_actual_variable_range() to take a noticeable amount of time, if you've
got enough such dead entries.  But the constant factor is much much better
than before, since all we need to do with each index entry is test its
"killed" bit.

We chose to back-patch commit fccebe421 at the time, but I'm hesitant to
do so here, because this form of the problem seems to affect many fewer
people.  Also, even when it happens, it's less bad than the case fixed
by commit fccebe421 because we don't get the contention effects from
expensive TransactionIdIsInProgress tests.

Dmitriy Sarafannikov, reviewed by Andrey Borodin

Discussion: https://postgr.es/m/05C72CF7-B5F6-4DB9-8A09-5AC897653113@yandex.ru

src/include/utils/snapshot.h

@@ -41,6 +41,7 @@ typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup,
  * * MVCC snapshots taken during recovery (in Hot-Standby mode)
  * * Historic MVCC snapshots used during logical decoding
  * * snapshots passed to HeapTupleSatisfiesDirty()
+ * * snapshots passed to HeapTupleSatisfiesNonVacuumable()
  * * snapshots used for SatisfiesAny, Toast, Self where no members are
  *	 accessed.
  *
@@ -56,7 +57,8 @@ typedef struct SnapshotData
 	/*
 	 * The remaining fields are used only for MVCC snapshots, and are normally
 	 * just zeroes in special snapshots.  (But xmin and xmax are used
-	 * specially by HeapTupleSatisfiesDirty.)
+	 * specially by HeapTupleSatisfiesDirty, and xmin is used specially by
+	 * HeapTupleSatisfiesNonVacuumable.)
 	 *
 	 * An MVCC snapshot can never see the effects of XIDs >= xmax. It can see
 	 * the effects of all older XIDs except those listed in the snapshot. xmin