feat: PLT-396: Add gevent pooler

[farioas]

Let's say you need to execute thousands of HTTP GET requests to fetch data from external REST APIs. The time it takes to complete a single GET request depends almost entirely on the time it takes the server to handle that request. Most of the time, your tasks wait for the server to send the response, not using any CPU.

The bottleneck for this kind of task is not the CPU. The bottleneck is waiting for an Input/Output operation to finish. This is an Input/Output-bound task (I/O bound). The time the task takes to complete is determined by the time spent waiting for an input/output operation to finish.

If you run a single process execution pool, you can only handle one request at a time. It takes a long time to complete those thousands of GET requests. So you spawn more processes.

However, there is a tipping point where adding more processes to the execution pool hurts performance. The overhead of managing the process pool becomes more expensive than the marginal gain for an additional process.

In this scenario, spawning hundreds (or even thousands) of threads is a much more efficient way to increase capacity for I/O-bound tasks. Celery supports two thread-based execution pools: eventlet and gevent. Here, the execution pool runs in the same process as the Celery worker itself. To be precise, both eventlet and gevent use greenlets and not threads.