added priority support for cluster state updates:

 * URGENT:
    * cluster_reroute (api)
    * refresh-mapping
    * cluster_update_settings
    * reroute_after_cluster_update_settings
    * create-index
    * delete-index
    * index-aliases
    * remove-index-template
    * create-index-template
    * update-mapping
    * remove-mapping
    * put-mapping
    * open-index
    * close-index
    * update-settings

* HIGH
    * routing-table-updater
    * zen-disco-node_left
    * zen-disco-master_failed
    * shard-failed
    * shard-started

* NORMAL
    * all other actions