Add ownersLock to protect concurrent access to node.owners between
GraphBuilder.processGraphChanges() (writer) and GC worker goroutines
reading in blockingDependents() and unblockOwnerReferences() methods.
Also fix concurrent reads in the HTTP debug handler (/graph endpoint)
for owners, dependents, beingDeleted, deletingDependents, and virtual
fields by using their respective thread-safe accessor methods.
