Unify TCP and UnixSocket APIs

[mpilquist]

Fixes #3560

This PR revamps fs2.io.net in a number of significant ways:

• Unifies support for IP and Unix sockets in to a single set of operations -- connect for clients and bind for servers, both taking the new com.comcast.ip4s.GenSocketAddress type, which is a supertype of SocketAddress[Host] and UnixSocketAddress
• Resultantly adds support for socket options on unix sockets
• Deprecates SocketGroup - there has never really been a great reason to manage explicit thread pools / async socket groups & the API was JVM only. With the new polling system, there's even less of a reason to ever use the explicitly managed socket groups.
• Deprecates UnixSockets as there's no utility provided that's not also provided by Network now
• Deprecates client, server, serverResource in favor of connect, bindAndAccept, bind
• Introduces ServerSocket, which provides the bound address of the server socket, access to options, and a stream of client sockets.
• Introduces SocketInfo as a supertype of Socket and ServerSocket.
• Introduces address: GenSocketAddress on SocketInfo and peerAddress: GenSocketAddress on Socket, replacing localAddress and remoteAddress (each of type F[SocketAddress[IpAddress]].
• Deprecating isOpen on Socket which is a vestige from when there was a close method.