Improve concurrent delete of proxies

NXP-30010 helps with the concurrent deletion of different proxies for the same target, but one issue remains.

When a proxy is deleted, it changes the ecm:proxyIds of the target, which in turn triggers an update of all its remaining proxies (copy of the target data onto the proxy). This update should be empty, as the target hasn't actually changed, but the code currently copies the ecm:systemChangeToken of the target onto the proxy, which, when two or more of these happen concurrently, triggers a (useless) proxy update that collides with the concurrent deletion of the proxy.