Adopting an existing, pre-bound server-socket channel

[PawelStroinski]

The use-case is to pass a server-socket channel, e.g. obtained by calling JVM's System/inheritedChannel. In this case, Aleph/Netty is not responsible for opening the channel nor closing it. It should simply listen on it. In other words, maintaining the connection sockets is still Aleph/Netty's responsibility, but not maintaining the listening socket if it has been passed to it.

• In the server startup part, this has been implemented as per Netty's Norman Maurer's advice found here.

• In the server shutdown part, Aleph is not shutting down nor closing the passed socket, because in practice it's owned by a process that passed it to Aleph process, and it's that parent process' responsibility to reuse or close it after Aleph process exits.

In terms of validation and fallback, for comparison, this used to be Jetty's approach to adopting inheritedChannel at version 8.

• Jetty automatically falls back to opening its own socket (with a warning) if the inheritedChannel is not supported or not present. In Aleph implementation, it refuses to start the server if the channel type is not supported, or the socket is not bound, but it silently falls back to an own socket if the passed channel is nil. As a result, it's Aleph user's responsibility to trigger an error if it expects an 'inherited channel' which is missing. Still, the user can expect that Aleph will fail early if a channel was actually passed, but it cannot be used.

• Unlike Jetty, Aleph doesn't itself call configureBlocking false on the channel because this is done by Netty during server startup.

[arnaudgeiser]

Code-wise it's looking great.
However, I would let @DerGuteMoritz decide whether or not this feature should land on Aleph (to me, it definitely makes sense).