RegionMover should deal with unknown (split/merged) regions

For a cluster with very high load, it is quite common to see flush/compaction happening every minute on each RegionServer. And we have quite high chances of multiple regions going through splitting/merging.

RegionMover, while unloading all regions (graceful stop), writes down all regions to a local file and while loading them back (graceful start), ensures to bring every single region back from other RSs. While loading regions back, even if a single region can't be moved back, RegionMover considers load() failure. We miss out on possibilities of some regions going through split/merge process and the fact that not all regions written to local file might even exist anymore. Hence, RegionMover should gracefully handle moving any unknown region without marking load() failed.