This picks up where cfd95340b1 left off.
The goal is to ensure that the butler has completed all
PostTransportStop related tasks and won't meddle with transport
after exporting has started.
Previously this could happen, because realtime_stop() queues
PostTransportStop and the butler is sommoned after every
export process cycle.
Since 61e7f3176b the butler keeps calling non_realtime_stop()
every time it is woken up, until TFSM comes around and unsets the
flag in the process callback.
4f3a95a1da