Fix an issue where a message is not delivered when an application reconnects (after receiving a push) using the same transport address as the one that was previously used (but no longer working).
Fixing this required to make some changes in ForkContext::onNewRegister() that exhibited a severe leak of RegistrarDb subscriptions, because the OnContactRegisteredListener passed to RegistrarDb::unsubscribe() was in many cases null, by design of things.