util: observable proxy objects for sockets We previously introduced proxy objects and observers for file objects to help implement low-level tests for the SSH wire protocol. In this commit, we do the same for sockets in order to help test the HTTP server. We only proxy/observe some socket methods. I didn't feel like implementing all the methods because there are so many of them and implementing them will provide no short term value. We can always implement them later. # no-check-commit because we implement foo_bar methods on stdlib types Differential Revision: https://phab.mercurial-scm.org/D2721