[FileBackend] Added support for concurrent file write operations.

[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