function on (event, handler) {
  if (!globalConnectionHandlers[event]) globalConnectionHandlers[event] = []
  globalConnectionHandlers[event].push(handler)

  if (globalConnection) globalConnection.on(event, handler)
  return globalConnection
}
Attach event handler to global connection pool.

@param {String} event Event name.
@param {Function} handler Event handler.
@return {ConnectionPool}