factor net into modules (#1831)

Summary:
Pull Request resolved: #1831

This is a straightforward refactor of `net` into `net::server` and `net::client`. More to come.
ghstack-source-id: 322824633

Reviewed By: shayne-fletcher

Differential Revision: D86800094

fbshipit-source-id: dc5ce0efb473a381870752c8c92a9d836af4a649

Author: mariusae