This change enables support for modern CDS in ContactsUpdater. It
accomplishes this by making the two CDS operation conform to a single
protocol, and then interacting with a generic interface.
This also makes some minor organizational changes to the contact
discovery operations. There's more that I want to do here but this
should work for now. This also fixes a bug where cancelled operations
might cause us to unregister all of our SignalRecipients. There's a
stopgap in place for now, but a more robust fix will come later.
Also updates the staging CDS enclave ID.
1ceffed228