Remove completed threshold in continuous aggregate materializer

[erimatnor]

The new refresh framework for continuous aggregates removes the need for the completed threshold. Progress in materialization will instead be tracked only in the cagg invalidation log.

Tracking materialization progress without the completed threshold necessitates initializing the cagg invalidation log with an infinite invalidation entry: [ -Infinity, + Infinity].

Adding such an entry allows tracking exactly the regions that are invalid and by inverse the regions that are valid. The information that was tracked by the old completed threshold will exist as the entry [ X, +Infinity ] in the cagg invalidation log. The completed threshold table can be removed.

However, with the new refresh framework, the completed threshold doesn't have much purpose as there is no distinction made between invalid and previously not materialized regions. Instead, a background policy an be free to decide, using heuristics, which regions within a refresh window to prioritize during each refresh.