EMFUtil.AbsoluteURIPreservingURIHandler should not be a singleton

[vsennedot]

A URIHandler should not be a singleton as its baseURI field can be set. Consequently, when multiple load/save operations occur in different threads, the baseURI field could have the value of another resource, leading to an undesirable deresolve result.

Although this can be avoided using the "org.eclipse.emf.cdo.common.model.EMFUtil.CONVERT_TO_RELATIVE_URIS" property, I don’t think it should remain a singleton in any case.

[claesrosell]

I have ran into this bug when we tried to upgrade to the latest version of CDO.
We have multiple meta-models with cross-references and the readCDOPackageUnit() method of CDODataInputImpl.java simply fails to ready packages when multiple models are read at the same time.
We to can avoid it by setting org.eclipse.emf.cdo.common.model.EMFUtil.CONVERT_TO_RELATIVE_URIS to true but I would really like to use the default behaviour since I suspect that the new model migration stuff works better with it.

I will provide a PR for this

[estepper]

Please give me a few days more to catch up with my backlog. I hope to find time next week...