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