feat: add env vars to configure the runtime threadpool size and name

[jbr]

I read the thread at #690 and this seemed like an easy change to make.

I tried to find a good balance between accurate-but-verbose (ASYNC_STD_RT_POOL_SIZE, ASYNC_STD_RT_THREAD_NAME) and concise-but-generic (RT_THREADS, RT_NAME).

Would people want to be able to optionally include the thread number in the thread name? I'm not certain why they would want to, but I believe I've seen that in other threadpools. If so, the thread name variable might include some substitution placeholder.

docs:

[dignifiedquire]

• I think we would want to document this in the top level docs and the task docs.
• This looks good from my side, but it changes things, such that we now allog configuration, would like to get some feedback from @skade and @yoshuawuyts before merging

[yoshuawuyts]

This matches what we've been talking about for a while. LGTM!

[jbr]

Added docs and a screenshot.

I was unsure if the documentation should explain the difference between parallelism and concurrency in an async environment, or trust that the reader understand that ASYNC_STD_NUM_THREADS isn't the ceiling on concurrency

[dignifiedquire]

Thank you @jbr lets get this in and see how it works out