Correct replicator states

[ricellis]

Checklist

• 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) or test/build only changes
• Completed the PR template below:

Description

Correct the list of valid replicator states

Fixes #464

Approach

• Add the failed state to follow_replication since this is another condition under which it should exit in CouchDB 2.1+.

Schema & API Changes

follow_replication exits for the error state even though that is a retryable state in CouchDB 2.1+ systems, it was existing behaviour for Couch < 2.1. Using the error state as an exit condition is retained so the code continues to operate in the same way as it has since launch when interacting with CouchDB >=2.1 based servers.

Behaviour change for the failed state which should have ended follow_replication but didn't, this is a fix.

Security and Privacy

• "No change"

Testing

The replicator tests were missing some other states introduced in CouchDB 2.1 scheduler.

• Modified existing tests because they did not have the complete list of replicator states for CouchDB 2.1+ and intermittently failed as a result.

Monitoring and Logging

• "No change"