Suppress the performance checker warning by being sneaky #2620
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Motivation:
Users are receiving thread performance checker warnings when they create MTELGs. This is largely unnecessary: MTELGs are not typically created on the fly but instead once, at program startup. That means the issue here isn't really all that profound.
While we could work around it by substantially rearchitecting the event loops (see #2618), an easier fix is to observe that the thread performance checker doesn't fire when we use condition variables.
Modifications:
Remove the use of the DispatchGroup and replace it with a condition variable.
Result:
Simpler code and no thread performance checker warning.
Replaces #2618.
Resolves #2223.