Summary:
This fixes a two problems:
* A mistake in `KeyedMutex.runExclusive`.
* Logic about saving a document to s3 when the document is found to match what is already there.
`HostedStorageManager.flushDoc` could get caught in a loop if a document was uploaded to s3 and then, without any change to it, marked as dirty. Low level code would detect there was no change and skip the upload; but then the snapshotId could be unknown, causing an error and retries. This diff fixes that problem by discovering the snapshotId on downloads and tracking it. It also corrects a mutex problem that may have been creating the scenario. A small delay is added to `flushDoc` to mitigate the effect of similar problems in future. Exponential backoff would be good, but `flushDoc` is called in some situations where long delays would negatively impact worker shutdown or user work.
Test Plan: added tests
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2654
e30d0fd5d0