In FileBackend:

In FileBackend:
* Added getFileSize()/getFileStat() functions. Refactored some functions to use the stat function for better reuse and caching/consistency.
* Refactored streamFile() to allow for subclasses to avoid local file copying with less duplication. Also make last-modified check actually work since we always get the timestamp of the original file.
* Renamed 'ignoreErrors' parameter to 'force'.
In FileBackendMultiWrite:
* Simplified how read ops are done (use 'master' backend for consistency).
* Added consistency check to doOperationsInternal() to check if the files are synced.
* Various fixes after testing.
In StreamFile:
* Split out prepareForStream() function from stream() in StreamFile for code reuse.
In FileBackendTest:
* Properly cover FileBackendMultiWrite in tests.
* Various test improvements.