RATE LIMITS & IS CRITICAL PRIORITY
Before this change, there was a notion of “critical” discovery tasks
that had their own rate limit, and that rate limit was used for
UUIDBackfillTask. In this change, that logic has been generalized to
consider a separate rate limit for additional request types.
QUEUE PRIORITIES
All of the discovery operations were updated to use `.userInitiated` as
their priority. Before this change, the `UUIDBackfillTask` and message
sending flow used lower priorities. However, both of those should use
higher priorities -- the former blocks receiving messages and the latter
blocks outgoing messages. This change allows the code to be simplified.
TESTS
This removes most of the existing `UUIDBackfillTaskTest` test cases
(they were commented out) and all of the `ContactDiscoveryTaskTest` test
cases (which were for the rate limiter, which has been moved elsewhere).
It introduces a few new tests. These aren’t meant to be exhausive;
instead, they’re meant to hit most of the code in the new classes and a
few specific edge cases that aren’t likely to be hit in normal use.
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