MB-31664: Reapply - Switch over to not using backfill queue

The long-term plan is to remove the backfill queue.  However, to gain
confidence in the change prior to its removal we are disabling via the
use of a switch.

This switch over was originally done in
commit 5968b4b
(see http://review.couchbase.org/#/c/101275/) however it had to be
reverted in commit 3386b90
(see http://review.couchbase.org/101684) because it was discovered that
for a backfill, the checkpoint in the checkpointManager could become so
large that we end up hitting an OOM condition.

With the introducion of the ability to expel items from a checkpoints
where all cursor have past, this should no longer occur.  Therefore it
should be safe to disable the use of the backfill queue.

If after a period of time no issues are seen, the backfill queue and
associated switch will be removed.

Change-Id: I188cfbb48edbd4cf7279fa32f7933722e2d9577a
Reviewed-on: http://review.couchbase.org/105643
Tested-by: Build Bot <[email protected]>
Reviewed-by: Jim Walker <[email protected]>

Author: owend74

engines/ep/configuration.json

@@ -350,7 +350,7 @@
             }
         },
         "disk_backfill_queue": {
-            "default": "true",
+            "default": "false",
             "descr": "True if we want to use the dedicated backfill queue for the first disk snapshot",
             "dynamic": true,
             "type": "bool"