[FileBackend] Added support for concurrent file write operations.

• FS backends work via popen() and basic shell commands.
• Swift backends use the custom SwiftCloudFiles async features.
• Refactored storagePathsRead()/storagePathsChanged() to normalize the paths for correct dependency detection in FileOpBatch.
• Cleaned up SwiftFileBackend exception handling to make debugging easier.
• Added a quick and dirty performance testing script.
• Updated unit tests to include a run with parallelize=implicit.
• Improved file test failure output a bit.

Change-Id: I6a5ed743c30c598e0dd7301dbdb3631c460332fd