Use available processor number for default thread count in consumer work service

[acogoluegnes]

The current value is Runtime#availableProcessors() x 2, but it was set a long time ago and is quite high now considering the number of cores available nowadays.

Setting the value to Runtime#availableProcessors() should be reasonable for most workloads.

[acogoluegnes]

Original mailing list post: https://groups.google.com/g/rabbitmq-users/c/q-FQ4xCIEnU/m/4LcaRg8_AwAJ.