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When calling the `cagg_watermark` function to get the watermark of a Continuous Aggregate we execute a `SELECT MAX(time_dimension)` query in the underlying materialization hypertable. The problem is that a `SELECT MAX(time_dimention)` query can be expensive because it will scan all hypertable chunks increasing the planning time for a Realtime Continuous Aggregates. Improved it by creating a new catalog table to serve as a cache table to store the current Continous Aggregate watermark in the following situations: - Create CAgg: store the minimum value of hypertable time dimension data type; - Refresh CAgg: store the last value of the time dimension materialized in the underlying materialization hypertable (or the minimum value of materialization hypertable time dimension data type if there's no data materialized); - Drop CAgg Chunks: the same as refresh cagg. Closes #4699, #5307