(hopefully) fixes export randomly stalling on windows:
dequeue_request() was a single request (no queue) on Windows.
Butler::queue_request() is called
-> Butler goes to work..
-> while working, another request is queued
-> butler never sees this
-> deadlock
during Freewheeling/Export wait_until_finished()
waits for the 2nd request to be handled, and never returns.
9a30bfd0c6