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## Overview
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what problem it solves or how it makes things better. -->
The PR is aimed to provide downgrade functionality in case there is need to
downgrade database which was modified by builds with clustered purge feature.
After database is modified by builds with clustered purge feature, the disk
version is bumped to 7. Also, there are two newly introduced trees:
`PurgeTreeState` and `PurgeSeqTreeState`.
Once downgrade function is called, the disk version is back to the latest
version of current build. Also, `purge_seq` and `purged_docs` are re-introduced
again.
PS: current PR is based on commit `28ba48da5877a5d471253c1b587af7e3b3121fd9`
which doesn't include clustered purge feature.
## Testing recommendations
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Does it provides any behaviour that the end users
could notice? -->
TBD
## Related Issues or Pull Requests
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repositories please put links to those issues or pull requests here. -->
https://github.com/apache/couchdb/pull/1370
https://github.com/apache/couchdb/issues/1352
## Checklist
- [X] Code is written and works correctly;
- [ ] Changes are covered by tests;
- [ ] Documentation reflects the changes;
[ Full content available at: https://github.com/apache/couchdb/pull/1616 ]
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