code: function replicateDb (source, target, opts0, callback0) {
      const { opts, callback } = getCallback(opts0, callback0)
  
      // _replicate
      opts.source = _serializeAsUrl(source)
      opts.target = _serializeAsUrl(target)
  
      return relax({ db: '_replicate', body: opts, method: 'POST' }, callback)
    }
docs: http://docs.couchdb.org/en/latest/api/server/common.html#post--_replicate