blitz: Use MetadataRoot in MetadataStore implementations

This PR requires https://github.com/openmicroscopy/bioformats/pull/505 merging into the bioformats submodule.

OMEROMetadataStoreClient:
· getRoot returns null, rather than pixelsList
· getPixelsList returns the pixelsList (note: there are no users of this, or as far as I can see of the old getRoot version, so this could be dropped)
· Added setRoot variant which uses MetadataRoot

OmeroMetadata.java:
· Drop getRoot, since the DummyMetadata default implementation appears adequate here