Set max_workers parameter to ThreadPoolExecutor in AerJob

[hitomitak]

Summary

To fix #244.

Details and comments

Add max_workers parameter to ThreadPoolExecutor in AerJob. If this value is not set, AerJob creates threads endlessly.

[atilag]

So after talking with @hitomitak, we have decided to put max_workers = 1. That makes a lot of sense since the C++ part of Aer should be using all the CPU cores at their maximum capacity because of the nature of the simulation (CPU Bound). So we shall not allow more than 1 instance running locally, otherwise the performance will suffer, especially with larger simulations. For short simulations I guess that the only way to check that this hypothesis is true, is by testing... in the meantime, we are setting max_workers to 1.