Improve OTP shutdown behavior for consumers

[nsweeting]

Summary

Improves the consumer shutdown sequence to follow proper OTP conventions. Previously, incorrect child spec types, missing consumption cancellation, and lack of graceful drain logic meant that a SIGTERM (e.g. k8s pod termination) would result in abrupt process kills rather than an orderly shutdown.

Problems Fixed

• Consumer.Supervisor was typed as :worker with a 5s shutdown — the entire consumer subtree had only 5 seconds before being :killed. Now correctly typed as :supervisor with :infinity shutdown, allowing children to drain properly.
• No consumption cancellation during shutdown — the broker continued delivering new messages while workers were shutting down. Now calls AMQP.Basic.cancel first.
• Workers stopped sequentially — on an N-core machine with N workers, sequential DynamicSupervisor.stop calls could easily exceed the shutdown budget. Now stops all workers in parallel.
• Executer had no terminate/2 — when shut down externally, the spawned message-processing process was killed with no opportunity to finish. Now uses Task.async + Task.shutdown/2 to give in-flight work a 5s grace period before escalating, and nacks with requeue: true as a safety net.
• No explicit shutdown timeouts — all processes used OTP defaults (5s). Now uses a proper timeout chain: Consumer.Supervisor (:infinity) > Consumer.Server (30s) > Workers/Executers (25s).

Corrected Shutdown Sequence

SIGTERM
  -> Consumer.Supervisor gets :shutdown (timeout: :infinity)
    -> Each Consumer.Server gets :shutdown (timeout: 30s)
      -> terminate/2:
        1. AMQP.Basic.cancel — stop new message delivery
        2. Stop all Workers in parallel (25s timeout)
           -> Each Executer.terminate/2:
              a. Task.shutdown(task, 5_000) — grace period for in-flight work
              b. Safety-net nack with requeue: true if not completed
        3. AMQP.Channel.close — clean channel shutdown
  -> Producer.Pool, Topology.Server, Connection.Pool shut down after

Changes

• lib/rabbit/broker/supervisor.ex — Consumer.Supervisor child spec gets type: :supervisor
• lib/rabbit/consumer/supervisor.ex — Consumer.Server child specs get shutdown: 30_000
• lib/rabbit/consumer/server.ex — Add cancel_consumer/1, parallel stop_workers/1
• lib/rabbit/consumer/executer.ex — Refactor to Task.async, add terminate/2 with grace period and safety-net nack, shutdown: 25_000, completed state tracking
• mix.exs — Version bump to 0.22.0