[HBCK2] MasterRpcServices assigns method should try to reload regions from meta if the passed regions isn't found under AssignmentManager RegionsStateStore

Although HBCK2 related, this is a master side improvement. On situations where regions are missing in META, any online fix tool such as the one being implemented in HBASE-22567 would require a further master restart to get RegionsStateStore reloaded from META, so that master can be aware of the newly re-added regions. 

After regions are re-added to meta in CLOSED state, it should be possible to bring those by simply invoking hbck2 assigns command. But before MasterRpcServices.assigns submits an Assign procedure, it validates first if the given region is available on AssignmentManager.RegionsStateStore. The current patch reloads meta on  MasterRpcServices.assigns if the given region is not found on the first lookup, then try a new lookup again before giving-up on region assignment.