couchdb: add support for version 2.0.0

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.