FMonitoredProcess does not properly support -nothreading mode

For single threaded mode, we need to update FMonitoredProcess to handle processing in a single threaded fashion. Currently, we are continuing before the process has completed, which could result in major stability issues

We worked around an infinite loop in this class, but the underlying behaviour is still broken. See discussion on the UDN thread

Run the editor with -nothreading in a debugger
Break in FMonitoredProcess::Launch() and IsRunning()
Notice that IsRunning returns false immediately, before the process has completed