fix #10703: download pre-FS original metadata in FS

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/10703. To test,

1. run a local 4.4 server
1. import files with metadata, e.g., from `ome/data_repo/test_images_metadata/`
1. upgrade to 5.0, using `sql/psql/OMERO5.0DEV__6/OMERO4.4__0.sql`
1. import other files with metadata
1. check that client "download original metadata" works okay for both pre-FS and FS images, both global and series metadata
1. check that `OriginalMetadataRequestTest.testMetadataParsing()` passes.

(Lines may be re-ordered but ought still be under the correct sections.)

I first tried using `org.apache.commons.configuration.HierarchicalINIConfiguration` to parse the file but, for our kind of INI-format, it turned out to be more trouble than it was worth.

--no-rebase as 5.0-specific