Throttle repeated cloud proxy rebuilds

[yophilav]

Summary

• Five-second per-device cooldown: outcome-based failed or rapidly inactive cloud proxy creation is throttled per device; initial creation and the first replacement remain immediate.
• Monotonic timing: cooldown elapsed time uses Stopwatch.GetTimestamp() with Stopwatch.Frequency arithmetic instead of wall-clock time.
• Atomic active and handoff state separation: normal lookup sees only the active connection, while a separate preserved connection is available only for a pending token handoff.
• Generation fencing and serialized removal: per-device connection state uses a generation counter so stale creation or update results cannot overwrite a newer removal.
• Token-waiter atomic state and stale callback suppression: the TaskCompletionSource token waiter is atomically published and cleared; cancellation suppresses stale TokenNearExpiry callbacks.
• Connection cleanup: canceled, displaced, superseded, and invalidated connections are removed from state and closed.

Fixes #7535

Testing

• ConnectionManagerTest and RetryingCloudProxyTest: 34 passed
• ClientTokenCloudConnectionTest: 9 passed