- Removes ContactsUpdater singleton. It wasn't really tracking much
state anyway
- Introduces ContactDiscoveryTask. This creates a
ContactDiscoveryOperation (modern or legacy) and updates the
SignalRecipients database on completion. Returns a promise.
- Begin migrating away from operation queues
PromiseKit is weird about priority propogation, so this shouldn't be
used yet. Since all then closures are asynced, priorities don't
propogate down to underlying queues. This means the a
ContactDiscoveryTask on the main queue will not boost the priority on
the ModernContactDiscoverOperation's owned operation queue
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