Option to parallelize restoreStoreFile in RestoreSnapshotHelper#cloneRegion

In one of the MOB case, we found that cloneSnapshot of a MOB enabled table took very long time. The reason is that for the MOB region, there are lots of MOB files. It took long time to clone the MOB region. In order to speed up cloneSnapshot, we will provide an option to parallelize restoreStoreFile in RestoreSnapshotHelper#cloneRegion, so MOB can take advantage of it. For non-MOB tables, it will keep the current behavior.