AMQP-422 PublisherCallbackChannel: postpone close

JIRA: https://jira.spring.io/browse/AMQP-422

Add `stopSemaphore` and `closeTimeout` option to the `PublisherCallbackChannelImpl`
to postpone physical `close` until timeout or handlong for all pending confirms

Decrease `@Repeat` for `RabbitTemplatePerformanceIntegrationTests` for better build performance
We should make this a final field rather than create a new one for each `close()`.