BackendSessionService = function(app) {
  this.app = app;
}
Service that maintains backend sessions and the communication with frontend
servers.

BackendSessionService would be created in each server process and maintains
backend sessions for current process and communicates with the relative
frontend servers.

BackendSessionService instance could be accessed by
`app.get('backendSessionService')` or app.backendSessionService.

@class
@constructor