Fix custom write checkpoints

[rkistner]

Fixes a regression from #276. The bug is not any release, so not adding a changeset.

#276 refactored custom write checkpoints to be flushed together with other changes when using MongoDB storage, to make it easy to associate an op_id with it. The bug is that if there were no other changes to flush, the write checkpoints weren't flushed either. This fixes it, and adds a test to reproduce.

This also adds a log message to indicate when custom write checkpoints are being flushed (but no details included yet).

[changeset-bot]

⚠️ No Changeset found

Latest commit: a07a47c

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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