ServerBuilder and listening

[carl-mastrangelo]

Currently, the standard way to listen on a socket is to call addPort(), but from recent requests, this API is too limited. I would like to explore some ideas for expanding this functionality. Specifically:

1. Make a standard way to add multiple ports to a single server.
2. Make it possible to listen on things other than ports, in particular SocketAddresses.
3. Make it possible to add more ports / sockets after a Server has been built.
   1. This would effectively allow a server to add ports after start()
   2. Maybe allow servers to gracefully close ports they started on.
   3. Support having no listening ports on the builder. This would mean adding a ServerBuilder ctor that has no ports at all.

This issue is more exploratory rather than mandatory. Right now the request is just to listen on multiple ports, but we should be careful to not add overlapping APIs (such as on both Server and ServerBuilder).

cc: @ejona86

[carl-mastrangelo]

Related: #5067

[ejona86]

For things like this we should be clear what the use-case is.

I'm aware of a use-case to listen on multiple addresses, all for the same port. Binding to a particular address is semi-common and it makes sense that you may need to bind to more than one (say, IPv4 + IPv6) very commonly.

[carl-mastrangelo]

A use case (that I think you brought up) is listening on both a port and a inprocess address. Another use case would be listening solely on loopback for security reasons.

[carl-mastrangelo]

Another use case: using different credentials per listening port, or different socket opts per port.