Defining aliases for data center names

When someone configures a multi-DC cluster, they have to define data center IDs to indicate where processes are located. Someone may want to change those IDs without moving to a new data center to support new conventions in their organization. I don't know that we have a clean migration story for this. Changing the DCs in either the start commands or the database configuration without changing both would take the database unavailable. I think the cleanest solution today would be to bring up new processes and treat it like a migration to a new DC, but this is fairly resource-intensive and can hit problems because of the limit of two active regions. As an alternative, could we support defining aliases for data center IDs to treat two different data center IDs as being equivalent for replication purposes?

Start by reviewing how data center IDs are supplied in the process start commands and database configuration, then trace how replication treats those IDs. The issue does not name files or tests; done would require a decided migration or alias design that avoids unavailable databases and handles the two-active-region limit.

Written by the indexing model from the issue text.