20183: Move update_priority tests. Fix updater starvation.

20183: Move update_priority tests. Fix updater starvation.

Previously (in the Rails implementation) the "find containers that
have priority=0 but need priority>0" query was returning all
containers with active (committed, priority>0) requests. However, it
is possible for all such requests to have parent
containers (requesting_container_uuid) to have priority=0, in which
case Container.update_priority!() leaves the container priority at 0.

With the controller implementation, this was manifesting as lack of
progress, and warnings in logs.

This commit fixes it by not including such containers as needing
priority>0.

With the previous Rails setup, this could cause infinite recursion:
the updater thread called update_priority!(), which left
priority=0 but still triggered an after_commit hook, which
(if the updater thread had already been running for >5s) started a new
updater thread, which called update_priority!() on the same container,
etc.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>