Fix ROOT-6445 (TThread initialization prevent TApplication::Terminate() without user input)
Fix ROOT-6445 (TThread initialization prevent TApplication::Terminate() without user input)

if the TThread Timer frequency is less or equal to kItimerResolution, TUnixSystem::DispatchOneEvent i
can not exit and have its caller react to the other TTimer's actions (like the request
to stop the event loop) until there is another type of event.

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