Add ThreadPool in Legacy implementations of MasterStorage/ RegionStorage

Certain bulk operations can be and needs to be optimized by doing them in parallel with ThreadPool (e.g. ModifyRegionUtils.createRegions()). This is FS dependent and may not be required for other implementations. Instead of instantiating a ThreadPool per request we can have single instance of StorageThreadPool and support bulk operations.

Also add APIs for bulk operations: createRegions(), deleteRegions(), archive regions() etc.