mutations
Whenever an alter table occurs, the mutations for the just altered table
are sent over to all of the replicas from the coordinator.
In a mixed cluster the mutations should be adapted to a specific version
of the schema. However, the adaptation that happens today doesn't omit
mutations to newly added schema tables, to be more specific, mutations
to the `computed_columns` table which doesn't exist for example in
version 2019.1
This makes altering a table during a rolling upgrade from 2019.1 to
2020.1 dangerous.
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