Add support for CouchDB 2.1.0

[flimzy]

CouchDB 2.1.0 was recently released. Kivik should support 2.1.0 fully. See What's New in 2.1.0

This is waiting on an official Docker image (see apache/couchdb-docker#23).

The following CouchDB 2.1.0 changes may require changes in Kivik:

• New replication scheduler [works out-of-box with work-around, 2.1 Replication should not require update_docs=true #215 for complete fix]
• Replication filter now supports using the body of the replication request for Mango queries.COUCHDB-2988 [No change needed]
• Add support for reporting Too Large errors. COUCHDB-2992 [Added status code in Add support for CouchDB 2.1.x #214]
• “stale=ok” option replaced with new “stable” and “update” options. COUCHDB-3036 [No change needed]
• New info in welcome message COUCHDB-3180 [Add support for CouchDB 2.1.x #214 & https://github.com/Add support for the new 'features' field in the welcome message couchdb#13 ]
• OAuth1 support removed [Doc changes only, in Add support for CouchDB 2.1.x #214]

Please add comments to this issue regarding any other relevant changes that should be made or considered for supporting CouchDB 2.1.0.

[flimzy]

At the moment, Kivik's replication appears to work with CouchDB 2.1 when update_docs is true. This is considered a work-around, and #215 will track fixing this more completely.

[flimzy]

#215 has now been resolved, so I'm closing this issue.