Defer the quit() of main loop.

The current impl. has race condition of TcpClient destructs before
TcpConnection calling TcpClient::removeConnection(), and TcpClient's loop_
member becomes wild pointer and assertInLoopThread fails because threadId_
is garbage value. Client::onDisconnect() makes the main event loop quits,
therefore destructs the TcpClient while TcpConnection connection callback
is half-way done in IO thread. The solution is to defer quit of main loop,
so TcpClient::removeConnection() finishes first.