Make LoadMonitor use $serverIndexes in the cache key

Make LoadMonitor use $serverIndexes in the cache key

* This avoids collisions of generic and custom LB group
  server list keys, which could cause warnings and errors.
* Remove $group param from scaleLoads(), which was unused
  and less robust for making keys anyway.
* Remove clearCaches() method, which only had one caller
  in a script that printed lag times in a loop. Its not
  worth keeping and having to pass in the server index
  list.
* Also guard scaleLoads() against recent server additions.

Bug: T147359
Change-Id: Idd15f0bebb68782fda36f483880cf7fe9673b940