<rdar://problem/13700260>

Avoid a deadlock when using the OperatingSystemPython code and typing "process interrupt". There was a possible lock inversion between the target API lock and the process' thread list lock due to code trying to discard the thread list. This was fixed by adding a boolean to Process::Halt() that indicates if the thread plans should be discarded and doing it in the private state thread when we process the stopped state.