Reuse Room IDs when first server in the list changes

[fredo]

Problem Description

When the known servers list changes such that the first server in the list is removed, the new first server will become the sync point for broadcast rooms for the rest of the list. Currently, this instance will create new rooms with fresh room ids and repoints the aliases to it. This causes an interval where servers are in different broadcast rooms and therefore out of sync. This was discovered during the testing phase for the new release.
This period of Raiden nodes not working correctly can be completely avoided if the new first server simply renames the alias for the broadcast rooms and reuse the room_ids. It should be a simple fix and remove all friction for this specific case which should happen rarely anyway.