[issue-719] Explicitly cancel outstanding checkpoints when checkpoints get stuck

[crazyzhou]

Change log description Add a cancelOutstandingCheckpoints call when the checkpoint hits MaxNumberOfCheckpointsExceededException

Purpose of the change Fixes #719

What the code does Add a cancelOutstandingCheckpoints call when the checkpoint hits MaxNumberOfCheckpointsExceededException

How to verify it ./gradlew clean build passes

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% :tada:

Comparison is base (cf2c995) 81.20% compared to head (68636ea) 81.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #720 +/- ## ============================================ + Coverage 81.20% 81.23% +0.02% - Complexity 572 574 +2 ============================================ Files 62 62 Lines 2719 2723 +4 Branches 232 232 ============================================ + Hits 2208 2212 +4 Misses 324 324 Partials 187 187 ``` | [Files Changed](https://app.codecov.io/gh/pravega/flink-connectors/pull/720?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pravega) | Coverage Δ | | |---|---|---| | [...pravega/connectors/flink/ReaderCheckpointHook.java](https://app.codecov.io/gh/pravega/flink-connectors/pull/720?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pravega#diff-c3JjL21haW4vamF2YS9pby9wcmF2ZWdhL2Nvbm5lY3RvcnMvZmxpbmsvUmVhZGVyQ2hlY2twb2ludEhvb2suamF2YQ==) | `92.18% <100.00%> (+0.52%)` | :arrow_up: |

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