schema tables: Remove mutations to unknown tables when adapting schema

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.

db/schema_tables.cc

@@ -783,6 +783,13 @@ future<std::vector<canonical_mutation>> convert_schema_to_mutations(distributed<
 
 std::vector<mutation>
 adjust_schema_for_schema_features(std::vector<mutation> schema, schema_features features) {
+    //Don't send the `computed_columns` table mutations to nodes that doesn't know it.
+    if (!features.contains(schema_feature::COMPUTED_COLUMNS)) {
+        schema.erase(std::remove_if(schema.begin(), schema.end(), [] (const mutation& m) {
+            return m.schema()->cf_name() == COMPUTED_COLUMNS;
+        }) , schema.end());
+    }
+
     for (auto& m : schema) {
         m = redact_columns_for_missing_features(m, features);
     }