Arvados

Partial fix for #20183.

We are seeing a deadlock/starvation issue where a container, whose crunch-run process has ended, cannot be finalized even with repeated attempts by arvados-dispatch-cloud (5m postgresql statement timeout), because its "select for update" is blocked by an UpdatePriority thread doing CPU- and memory-intensive things while keeping a database transaction open with uncommitted updates.

The idea here is to reduce the duration of open transactions in UpdatePriority threads.