tag shared and temp files with username

[acutkosky]

Description of changes:

1. Change StreamingDataset to make a temporary directory in tmp/<username>/streaming rather than tmp/streaming.
2. Change the shared memory prefix to <username>_<old_prefix>

These changes partition shared resources on a per-user basis to prevent permissions errors when multiple users are working on the same system.

Issue #429

• Fixes #429

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[knighton]

How can we do this while still keeping the local dir collision check across all users?