### ClickHouse release v21.3.15.4-stable FIXME as compared to v21.3.14.1-lts

#### Bug Fix
* Backported in [#25956](https://github.com/ClickHouse/ClickHouse/issues/25956): Fix extremely long backoff for background tasks when the background pool is full. Fixes [#25836](https://github.com/ClickHouse/ClickHouse/issues/25836). [#25893](https://github.com/ClickHouse/ClickHouse/pull/25893) ([alesapin](https://github.com/alesapin)).
* Backported in [#26141](https://github.com/ClickHouse/ClickHouse/issues/26141): Fix possible crash in `pointInPolygon` if the setting `validate_polygons` is turned off. [#26113](https://github.com/ClickHouse/ClickHouse/pull/26113) ([Alexey Milovidov](https://github.com/alexey-milovidov)).