code: function (mappings) {
      if (!mappings || !_.isObject(mappings)) {
        var err = new Error('Please pass a valid mapping values!');
        debug(err);
        throw err;
      }
  
      this.eventMappings = mappings;
  
      return this;
    }
docs: Define which values should be mapped/copied to the payload event. [optional]
  @param {Object} mappings the mappings in dotty notation
  {
  id: 'id',
  commitId: 'commitId',
  commitSequence: 'commitSequence',
  commitStamp: 'commitStamp',
  streamRevision: 'streamRevision'
  }
  @returns {Eventstore}  to be able to chain...