Weak reference targets added after the reference property are not indexed

with the introduction of a special index for (weak)references by OAK-1137, they are only indexed if a reference property is written, but not if a weak reference target is added later on.

see org.apache.jackrabbit.oak.jcr.ReferencesTest#testWeakReferencesAddLater that fails currently.

the only workaround is to re-set the weak reference property after adding the referenceable node.

looking at this regression, I don't think that the current approach of storing the back references is a good one as it does not work for this case. IMO a separate index of all reference properties, grouped by UUID of the target referenceable needs to be kept outside of the normal tree.