Reverts 49736b3a, large file copies can't avoid the RTT.
The parent stack must be blocked while FileService progresses, as unlike
the small file path, it does not make a snapshot of the (possibly
temporary) file passed by the action plug-in. So we need to keep that
file alive while the service runs.
Add a new integration test and a new soak test to cover both.
638b196a45