BackendSession = function(opts, service) {
  for(var f in opts) {
    this[f] = opts[f];
  }
  this.__sessionService__ = service;
}
BackendSession is the proxy for the frontend internal session passed to handlers and
it helps to keep the key/value pairs for the server locally.
Internal session locates in frontend server and should not be accessed directly.

The mainly operation on backend session should be read and any changes happen in backend
session is local and would be discarded in next request. You have to push the
changes to the frontend manually if necessary. Any push would overwrite the last push
of the same key silently and the changes would be saw in next request.
And you have to make sure the transaction outside if you would push the session
concurrently in different processes.

See the api below for more details.

@class
@constructor