SessionService = function(opts) {
  opts = opts || {};
  this.singleSession = opts.singleSession;
  this.sessions = {};     // sid -> session
  this.uidMap = {};       // uid -> sessions
}
Session service maintains the internal session for each client connection.

Session service is created by session component and is only
<b>available</b> in frontend servers. You can access the service by
`app.get('sessionService')` or `app.sessionService` in frontend servers.

@param {Object} opts constructor parameters
@class
@constructor