Only unregister the listener when eph is true

[evomassiny]

Thank you for this repo, it helped me a lot to grasp the basics of asynchronous servers under linux !

Nevertheless -on my config- after any request, the current version of this repo will panic at the end of the connection, when dropping the AsyncTcpStream, with this error:
epoll:ctl_del_rawfd:error: No such file or directory

My guess is, when a tcp stream is dropped, it's already been automatically unregistered from its Epoll file descriptor set, because of the use of the flag EPOLLONESHOT

A quick way to solve this is to check if the stream still belong to an Epoll file descriptor set, by looking at stream.eph flag.