code: function setMessagesRead(state, { threadID }) {
    return state.updateIn([threadID, 'messages'], messages => {
      return messages.map(msg => msg.set('isRead', true))
    })
  }
docs: Mark all messages for a thread as "read"
  @param {Immutable.Map}
  @param {Object} payload
  @param {GUID} payload.threadID