don't use substraction for comparison if datatypes can overflow
Added sort by field that have multiple values per document.

Closes #2634
Extended mv sorting integration test
Cleanup NumericFieldData. FieldData interfaces are reduced to long and double while internal
represenations still operate on the actual datatypes.
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