code: function combineChangesets (changesets) {
    const actions = changesets.map((changeset) => changeset.apply)
  
    const combinedAction = function () {
      const args = arguments
  
      return actions.reduce((promise, action) => (
        promise.then(() => action.apply(null, args))
      ), Promise.resolve())
    }
  
    return createChangeset(combinedAction)
  }
docs: Takes an array of changesets and returns a single changeset that contains
  all of the input changeset's DB operations.
  
  @param {Changeset[]} changesets
  @return {Changeset}