Support for dynamic concurrency

[manast]

We would like to explore the possibility to allow defining a concurrency setting that instead of being fixed for a Worker would be dynamic, based on the CPU usage of the worker, if the CPU is very high then the concurrency will be decreased, if it is under a given level it would be increased. These would settings that could be tweaked for every worker.
Here some information on how to achieve this CPU usage detection:
https://nodejs.org/api/perf_hooks.html#performanceeventlooputilizationutilization1-utilization2
https://nodesource.com/blog/event-loop-utilization-nodejs/