Stashing: naming and folder location

We have seen cases where node stashing failed due to RuntimeExceptions (OOM or MongoDB issues in Oak). In these cases, the tmp folder is not cleaned up. If the operation is retried many times, there'll be many of these.

Suggestion:

1. Do not use the root as default location,
2. Introduce an intermediary node that makes it clear that this is temp space created by FileVault.