NEXUS-5930: OBR shadow makes Nexus deadlock prone

Changes:
* marks the `AbstractShadowRepository#onRepositoryItemEvent` method with `AllowConcurrentEvents` annotation
* ups the method level where ObrShadow generates missing OBR data (to avoid deadlock)


Issue
https://issues.sonatype.org/browse/NEXUS-5930

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF11