Revised RootBeanDefinition's externallyManaged* Sets to rely on postProcessingLock

This allows us to avoid early initialization of footprint-heavy ConcurrentHashMaps, and reuses the existing postProcessingLock which will typically be active already when registerExternallyManaged* calls come in from the bean factory's post-processing phase.

Issue: SPR-11343
(cherry picked from commit a599b57)