`ALTER TABLE ... FETCH` and `ALTER TABLE ... ATTACH` queries were
disabled in the Replicated database engine, because it could cause
accidental duplication of data.
This enables these queries but without replicating them.
In the case of `FETCH`, the part will only be fetched on the server
where the query is issued.
Similarly, in the case of `ATTACH`, the attached part only needs to
be available on the server where the query is issued.
If the table itself is using one of the Replicated MergeTree engines,
the attached data is then replicated by the table engine itself,
without intervention of the database engine.
This change is meant to help with live backup/restore when using the
Replicated database engine, using FREEZE for backup and ATTACH for
restore.
