add priority support for cluster state updates:
|routing-table-updater|URGENT|
|shard-failed|URGENT|
|shard-started|URGENT|
|refresh-mapping|URGENT|
|cluster_update_settings|HIGH|
|reroute_after_cluster_update_settings|HIGH|
|delete-index|HIGH|
|update-mapping|HIGH|
|remove-mapping|HIGH|
|put-mapping|HIGH|
|close-index|HIGH|
|open-index|HIGH|
|_all others_|NORMAL|