Backport HBASE-14473 (Compute region locality in parallel) to 0.98

Elliott Neil Clark contributed a nice change on HBASE-14473 that scales calculation of locality balance cost to larger clusters. I'd like to bring this back to 0.98 for folks running it on larger clusters. The changes require a partial rewrite of RegionLocationFinder hence a backport issue.

The difference between this backport and RegionLocationFinder on later branches is we preserve the ability to change the expiration time of cache items with the configuration parameter "hbase.master.balancer.regionLocationCacheTime".

The difference between RegionLocationFinder in 0.98 before and after this change is the expiration time of cache entries will be set according to when they are written into the cache instead of from when they are last accessed, which seems better to me.