Use system temporary directory for default FileSystem path plugin

[fishcharlie]

We should use the os.tmpdir() string if the user doesn't pass in a custom path to the FileSystem plugin.

We should also append a custom string (ex. HeapStash_1) to the end of that string, and ensure that directory doesn't exist prior to creating it. That way you can have multiple HeapStash instances and they won't collide with each other.