Revert "Pylint cleanup for threadpool"

This reverts commit c93ca9aec2106a8dcc1404ae542c5889e8c2df15.
Revert "Remove some un-used variables"

This reverts commit 22ed10108ee1cf3b29af9220d6ba6fa57f532eff.
Revert "Remove "fire_event" from AsyncClientMixin, since this was only added to remove infinite recusion in the reactor-- which is now not calling this API"

This reverts commit 0b19ec7385e68914041f21959f50009a8a6f45a2.
Revert "Historically the recator has just called the "async" method of the runner and wheel clients, but this actually creates daemonized processes. In addition to creating a new daemonized process each event, the number of process it creates is unbounded, meaning that the reactor can easily use all available PIDs on a fairly busy master. In addition, there is no bound on the CPU that these are allowed to use (since they can create ALL the pids). This changes the reactor to create a threadpool for executing its master-side clients (runner/wheel). This threadpool has a configurable number of workers (max parallelism) and hwm (max queue size before dropping events)."

This reverts commit 2a8d2a462a5cecad3a4d7e88395f7a871a6bfdd9.

Conflicts:
	salt/utils/event.py
Merge pull request #18741 from terminalmage/revert-pr18264

Revert #18254