org.opendaylight.yangtools.yang.model.repo.api.SourceIdentifier object sharing

Large number of netconf sessions result in a large number of sources for the same module – which in turn explodes memory needed to just track availability – to some 10MB for 1000 sessions. Adding an ObjectCache for both SourceIdentifiers and for PotentialSchemaSources will allows us to bring those numbers down, increasing our ability to scale.