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This reverts commit 2a84df59 partially.
With a good index, requests should be fast enough to avoid locks.

Use following requests to update activity tables:

ALTER TABLE message DROP KEY processing_node_date, ADD KEY processing_node_priority_date (processing_node, priority, date);
ALTER TABLE message_queue DROP KEY processing_node_date, ADD KEY processing_node_priority_date (processing_node, priority, date);