Matthias Pohl

I rebased the branch to `master` after the base PR #24910 was merged to `master`.

[Observed CI failures](https://dev.azure.com/apache-flink/apache-flink/_build/results?buildId=60530&view=results) documented: * [FLINK-25453](https://issues.apache.org/jira/browse/FLINK-25453?focusedCommentId=17860817&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-17860817) for the `SqlGatewayE2ECase.testMaterializedTableInFullMode` * [FLINK-35722](https://issues.apache.org/jira/browse/FLINK-35722) for the `CoordinatorEventsToStreamOperatorRecipientExactlyOnceITCase.testCheckpoint`

@flinkbot run azure

[CI with AdaptiveScheduler enabled](https://dev.azure.com/apache-flink/98463496-1af2-4620-8eab-a2ecc1a2e6fe/_build/results?buildId=60616) was successful. I'm gonna go ahead and prepare this PR to be merged (i.e. remove the DO-NOT-MERGE commit).

@ztison I addressed your comments. PTAL

I rebased to most-recent version of the base PR #24911

I added another commit to PR #24911 where i introduce `CheckpointStatsTracker` as an interface. This allows us to get rid of `CachingSupplier` and makes it easier to test the main...

I addressed the last two comments and rebased the branch to most-recent version of parent PR #24911 . That way we also have the CI debug commit included

The [most-recent forced-push](https://github.com/apache/flink/compare/ef92f6afded2990a7338411c03fe4ee74ab025dd..92125a5fa97ffe42a08ad6900c7cd8243877a202) was to rebase to the most-recent version of base PR #24911 to prepare for the final commit reorg in this PR.

The [CI failure](https://dev.azure.com/apache-flink/98463496-1af2-4620-8eab-a2ecc1a2e6fe/_build/results?buildId=60613) doesn't seem to be related to this PR. I created FLINK-35748 to cover the topic.