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
Ha-ha! I see an issue:
in the end of `physicalClose()` we have 
```
finally {
	this.target = null;
}
```
and voila - `NPE`!