SHOW TABLES query times out after a minute

[zaychenko-sergei]

Depends on #986.

Note: after key blocks from #986 are implemented, this problem will unlikely need anything else specifically, but still should be tested and re-measured.

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Open a random dataset in demo environment, and queried show tables.
The query fails after 1 minutes with a server timeout:

Grafana tracing shows numerous schema queries for every dataset:

followed by endless repository read iterations:

Assuming this is caused with the following code fragment, that simply looks for SetDataSchema event in every dataset:

    #[tracing::instrument(level="info", skip_all, fields(dataset_handle = ?self.dataset_handle))]
    async fn init_table_schema(&self) -> Result<SchemaRef, InternalError> {
        let maybe_set_data_schema = self
            .dataset
            .as_metadata_chain()
            .accept_one(SearchSetDataSchemaVisitor::new())
            .await
            .int_err()?
            .into_event();

        if let Some(set_data_schema) = maybe_set_data_schema {
            set_data_schema.schema_as_arrow().int_err()
        } else {
            Ok(Arc::new(Schema::empty()))
        }
    }