Clear pendingValidatedCheckpoint on new checkpoints

[rkistner]

Fix regression from #661. What would happen:

1. "Could not apply checkpoint due to local data. We will retry applying the checkpoint after that upload is completed." -> sets pendingValidatedCheckpoint to checkpoint A.
2. Receive a data and checkpoint_complete message -> validates checkpoint B.
3. Crud upload complete -> queues a crud_upload_completed message.
4. Attempts this.applyCheckpoint(pendingValidatedCheckpoint) with checkpoint A. Checkpoint A is no longer valid, so this results in "Checksums failed", and re-downloading the data.

Was not part of a stable release, so no changeset required.

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[simolus3]

The implementation changes look good to me, but we'll need someone to check the node logging / testing changes I've added.

[rkistner]

Test looks good to me

[stevensJourney]

I was able to reproduce the checksum failure error previously when testing the YJS demo. After merging in these changes the issue was resolved for me. LGTM.