Synchronizes access to the proxy uri map in MapModelIndex

Access to MapModelIndex#updateIndexOnResourceLoaded might be called
during model loading (in an EMF write transaction), after model is
modified (probably inside an EMF transaction) and when the Eclipse File
System is updated (in workspace jobs triggered by workspace listeners).
The latter category can happen in parallel with the first two, making it
necessary to synchronize access to the internal data structures to avoid
issues in the code.

Change-Id: Idfd5993e1775322bb1eef98e000f7b77fb236caa
Signed-off-by: Balazs Varnai <balazs_varnai@mentor.com>
Signed-off-by: Zoltan Ujhelyi <zoltan.ujhelyi@incquerylabs.com>