Executing the commands inside a worker thread allows further improvements
to job queuing but mainly it fixes the problem of loading users when
listeners are allocated at runtime.
When a runtime listener was being created, it was allocated in the admin
thread whereas the listeners created at startup were allocated in the
"main" thread. This caused a minor difference in how administrative
functions were handled by the REST API and MaxAdmin. The only real problem
is that listener allocation depends on being done inside a worker thread
as it lazily initializes some resources.
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