Update to ImageJ 2.0.0-rc-2

This updates the version of imagej-common, to gain access to the revised
DatasetService#save method that returns the Metadata SCIFIO used for
writing.
OMEROService: revise how saver metadata is queried

Mark and I settled on a different approach for ImgSaver to provide
access to the Metadata used by the Writer. Rather than writing it into
the original ImgPlus's properties table -- which destroys the original
Metadata put there by the ImgOpener, can suffer from race conditions,
and be misleading with respect to its contents -- we instead have the
ImgSaver#saveImg methods return the Metadata. And ImageJ's
DatasetService#save does the same thing.
SaveToOMERO: make leaf label unique

It needs to not clash with OpenFromOMERO, or else ImageJ1 is sad.
Merge branch 'return-saver-metadata'

This fixes the way that we query the ImgSaver metadata. The
DatasetService#save method now returns a Metadata which we can
interrogate directly. This avoids race conditions.

See fde1727b36d5e0c199dfd502635db50d77c66bd5.