There is a change that a TCPServer may have accepted a new connection before the working thread pool started

[zieckey]

Time 1 : TCPServer::listener_::Listen
Time 2 : TCPServer::tpool_ is starting
Time 3 : A new connection is comming
Time 4 : TCPServer::HandleNewConn is invoked and we assert that
tpool_ must be running that would crash