searcher: listen for OS signals
Created by: keegancsmith
Currently we never start the signal listeners since server.ListenAndServe blocks. This means we currently have no signal handlers => instant shutdown on receiving a signal.
This was accidently introduced a month ago while refactoring graceful shutdowns across services. Before that we correctly listened for SIGINT only. See https://github.com/sourcegraph/sourcegraph/pull/27958