-Added new LocalityCostFunction and LocalityCandidateGenerator that
cache localities of every region/rack combination and mappings of every
region to its most local server and to its most local rack.
-Made LocalityCostFunction incremental so that it only computes locality
based on most recent region moves/swaps, rather than recomputing the
locality of every region in the cluster at every iteration of the
balancer
-Changed locality cost function to reflect the ratio of:
(Current locality) / (Best locality possible given current cluster)
Signed-off-by: Sean Busbey <busbey@apache.org>
Signed-off-by: Chia-Ping Tsai <chia7712@gmail.com>
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