Remove ManagedRawFileStoreI in favor of callbacks

ManagedRawFileStoreI was not being used which meant that
closeCalled was never invoked and therefore the proxies
in `ManagedImportProcessI.uploaders` were never cleaned.
This led to `ObjectNotExistException` being thrown on a
second loading of `proc.getUploader(i)`. Rather than add
logic to create the correct subclass of RepoRawFileStoreI,
a callback interface has been added which can be used for
any implementation.