[DISCUSS] Upgrade Snappy to 1.2.2

Hi,

this thread is for discussing the upgrade to the latest version of Snappy (1.2.2).

I prepared the snappy upgrade [here](https://github.com/apache/couchdb-snappy/tree/upgrade-snappy-1.2.2).
To test this Snappy version in CouchDB, I provide an additional branch with PR #5567.

What I test so far:

0. Install latest CouchDB (https://github.com/apache/couchdb/commit/91bd2edb6406ddd39d735548774a384b0ec3a46f)
1. Create a "shared data" directory for the dev runs and use a preconfigured ini file
```ini
[vendor]
name = The Apache Software Foundation

[couchdb]
uuid = fake_uuid_for_dev
database_dir = /path/to/shared/dir/data
view_index_dir = /path/to/shared/dir/data
file_compression = snappy

[smoosh]
state_dir = /path/to/shared/dir/data
```
2. Run this server with `./dev/run -aa:a -n1 -l snappy.ini`
3. Create a db with sample documents, do some changes, deletions, create views, compaction, ...
4. Shutdown CouchDB
5. Clone another CouchDB #5567 with the PR from above
6. Start this instance an try to load the database, views, documents

CouchDB should read the db and documents created by the older version of snappy.

Please test this and comment if this works for you.

Start with the upgrade branch and PR #5567, then follow the dev/run command using the provided snappy.ini configuration. Create sample databases, documents, views, changes, deletions, and compactions with the older version, then verify that the PR #5567 instance can load them successfully; report the compatibility results in this discussion.

Written by the indexing model from the issue text.