Version 2.0.0 is installed as a separate package called "couchdb2".
When setting the config option "package" attribute to pkgs.couchdb2, a
corresponding service configuration will be generated. If a previous
1.6 installation exists, the databases can still be found on the local
port (default: 5986) and can be replicated from there.
Note that single-node or cluster setup still needs to be configured
manually, as described in
http://docs.couchdb.org/en/2.0.0/install/index.html.
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