[WIP] Added handling for server side missing revs

[ricellis]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [ ] Added tests for code changes or test/build only changes
• [ ] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

If server side compaction removes revs between the iOS client fetching the changes feed and fetching the changes there are potential 404 not found conditions.

See #454

Approach

Since the 404 alone does not provide enough information to determine if it is transient (i.e. the document is yet to be consistent across all nodes) or permanent (i.e. the rev has been permanently removed) then we must assume the transient case in the first instance.

To deal with this any rev that gets a 404 from the bulk cases will be requeued for individual fetching. If the document is subsequently not found when an individual fetch attempt is made then it is considered a permanent failure.

This is approximately inline with the behaviour of the CouchDB replicator which performs a single retry after a configurable delay.

This change will not solve the problem for cases where only an individual fetch attempt is made, but all Couch 2.x support the _bulk_get endpoint and Couch 1.x is EOL so this fix should be suitable.

Schema & API Changes

• No API changes

Security and Privacy

• No change

Testing

TODO

Monitoring and Logging

• Added new log line to warn of permanent failures fetching a revision.

[ricellis]

This library is now end-of-life.