ChannelService = function(app, opts) {
  opts = opts || {};
  this.app = app;
  this.channels = {};
  this.prefix = opts.prefix;
  this.store = opts.store;
  this.broadcastFilter = opts.broadcastFilter;
  this.channelRemote = new ChannelRemote(app);
}
Create and maintain channels for server local.

ChannelService is created by channel component which is a default loaded
component of pomelo and channel service would be accessed by `app.get('channelService')`.

@class
@constructor