lib/one.js
function one (state, eventName, handler) {
  state.emitter.once(eventName, handler)

  return state.api
}
function one (state, eventName, handler) {
  state.emitter.once(eventName, handler)

  return state.api
}
function one ( state , eventName , handler ) { state . emitter . once ( eventName , handler ) return state . api }
binds event only once to handler

@param  {String} eventName   push, pull, connect, disconnect
@param  {Function} handler   function once bound to event
@return {Promise}
binds event only once to handler