A handle is the super class to all registry objects that are now served to users.
The handles never hold on to any "real" content of the object being represented.
A handle can become stale if its referenced object has been removed from the registry.
since: 3.1.
Monitor ensuring no more than one writer working concurrently.
Multiple readers are allowed to perform simultaneously.
This class was borrowed from org.eclipse.jdt.internal.core.search.indexing.