Issue Template

Title: Transport socket callback fd() needs to be generalized to an I/O handle for more generalized connections

Description:
Back towards the end of 2017 the transport_socket interface was introduced. In PR #2116, which added the transport_socket interface, @mattklein123 had the following comments:
"Not all transport sockets are going to have a fd. Can this be removed from the interface somehow? (I haven't yet looked in detail at how the rest of the code is factored). In now reading more of the PR, I guess this is due to the fact that fundamentally we are passing FDs around from listeners. I suppose at some point we may need to turn this into some type of generic handle but this is probably ok for now. Just thinking out loud."

As we prepare for the integration of VPP with Envoy, the issue of the generalizing of the fd is a central one, since that interface extends beyond that of a socket.