Added readAffinity flag to FileBackendMultiWrite
- A backend with this set (normally 1) will be used for non-latest reads. This can be used to prefer a local, replicated, backend instead of one farther away (for multi-DC setups)
- Note that listings still come from the master always.
Bug: T112708
Change-Id: Ic4bf4ba5a2c9ef78abd11dbd0d4b48c73cad6923