FS sync: UnregisteredFileException

This branch tries to better handle issues where files are created
on the filesystem outside of OMERO's knowledge. Rather than provide
methods which allow for reading both the DB state and the FS state,
the DB state is taken to be definitive, and discrepancies will be
handled on a per-method basis. In the case of file() and fileById(),
rather than just register a given file, a UnregisteredFileException
will be thrown.

This PR also adds a backdoor for administrators (RawAccessRequest)
which can be used to `touch file` or `mkdir [-p] dir` on the repo
for proper testing. All other test-only methods like create() have
been removed.

Primary criteria for testing this PR is making sure that the file*()
methods of PublicRepositoryPrx behave as expected. This should have
minimal impact on import.

/cc @knabar, @bpindelski, @mtbc, @jburel