Remove custom thread pools

[jasontedor]

Today, we allow the creation of arbitrary custom thread pools. I'm not convinced this is a useful feature and do think that it is a dangerous one. I propose that we remove the capability for custom thread pools.

[jpountz]

This would mean that plugins could not have dedicated thread pools anymore. They could use the generic threadpool instead, but this has the drawback that it is unbounded. Another option that was mentioned was to create a bounded thread pool that all plugins could share when they need a bounded thread pool. There were diverging opinions and we would like to hear @s1monw 's opinion before moving forward.

[jasontedor]

They could use the generic threadpool instead, but this has the drawback that it is unbounded.

The generic thread pool will be bounded after #17017.

[s1monw]

I think we should remove that option but need to be careful with the consequences. If plugins have long running tasks they might need a different infrastructure? Something like an internal long task pool we can only use from internal?

[s1monw]

@uboness what do you think ^^ I think it can be bound and rejects quickly such that we don't have too many long running tasks going on at the same time?

[s1monw]

removed the discuss label @uboness can you please comment

[jasontedor]

Closed in favor of #18613