The concept is needed by enterprise functionality, but in the hunt for globals this sticks out and should be removed.
This is also partially prompted by the need to handle the keyspaces in the above set special on shutdown as well as startup. I.e. we need to ensure all user keyspaces are flushed/closed earlier then these. I.e. treat as "system" keyspace for this purpose.
These changes adds a "extension internal" keyspace set instead, which for now (until enterprise branches are updated) also included the "load_prio" set. However, it changes distributed loader to use the extension API interface instead, as well as adds shutdown special treatment to replica::database.
Closes#13335
* github.com:scylladb/scylladb:
datasbase: Flush/close "extension internal" keyspaces after other user ks
distributed_loader: Use extensions set of "extension internal" keyspaces
db::extentions: Add "extensions internal" keyspace set
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