quitChannel method
退出本节点的某个消息推送组
Implementation
void quitChannel(EasyServerSession session, String cid) {
final channel = _websoketChannelMap[cid];
if (channel == null) return;
channel.remove(session.id);
session.quitChannel(cid);
if (channel.isEmpty) _websoketChannelMap.remove(cid);
logDebug(['quitChannel =>', session.info, cid]);
}