Improve perf of RegionLocationFinder.mapHostNameToServerName

RegionLocationFinder.getTopBlockLocations() is called multiple times during balancer. While profiling on a large table balance, mapHostNameToServerName() seem to take a lot of time. One of the maps is repeatedly created for each iteration, while we can just initialize it once.

Goes into both branch-1 and branch-2, although patches differ slightly.