Add single-active-consumer priority and cancel tracking to virtual transports

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What this task asks for

Add single-active-consumer semantics, consumer priority selection, cancel notifications, and lifecycle tracking to virtual transports. When a queue is declared with `x-single-active-consumer: True` in its queue arguments, at most one consumer receives messages at a time. `Channel.basic_consume` supports consumer priority via `x-priority` in consumer arguments (default 0) and an optional `on_cancel` callback. Consumer state must live in `BrokerState` (shared across channels), not per-channel. The `connection._callbacks[queue]` entry must dispatch to the correct consumer at delivery time, not simply store the last registered callback. When a higher-priority consumer registers on a SAC queue where a lower-priority consumer is active, the lower-priority consumer is demoted and its `on_cancel` fires. Transports with class-level `global_state` (memory, filesystem, pyro) must clear consumer state when a new Transport is created, since registrations must not leak across connections.