Evaluate & potentially replace `https://github.com/soheilhy/cmux`

### What would you like to be added?

etcdserver depends on https://github.com/soheilhy/cmux to receive both gRPC and REST connections on the same TCP listener/port. But the library isn't well maintained; note it hasn't been updated for more than 3 years at the time of writing this ticket. It also has issue working with HTTP/2, please refer to [soheilhy/cmux/issues](https://github.com/soheilhy/cmux/issues).

We should evaluate & summarize the problems the library has, and
- either try to find an alternative solution.
- or completely separate gRPC and REST/HTTP connections;

I expect someone with more network expertise could drive this effort.

### Why is this needed?

See above

Start by reviewing how etcdserver uses soheilhy/cmux to share a TCP listener, then examine the linked cmux issues for HTTP/2 problems. Done means documenting the library's problems and recommending either a viable alternative or separate gRPC and REST/HTTP connections, with implementation scope identified.

Written by the indexing model from the issue text.