Add live loader flag for configuring http endpoint for pprof.

[danielmai]

This change adds an --http flag to dgraph live to configure the HTTP port that is exposed for pprof (e.g., http://localhost:6060/debug/pprof). Live loader was already set up to expose localhost:6060 as the pprof endpoint, but this was not configurable. If two live loaders were running on the same machine, you'd see the following error and you could only profile one of the live loaders:

E0821 13:57:45.822777    7462 run.go:291] Error while starting HTTP server in port 6060: listen tcp 127.0.0.1:6060: bind: address already in use

• Add --http flag to live loader. This matches the flag name and description as dgraph bulk.
• Set localhost:6060 as the default value for the --http flag, which is the same addr:port used in live loader already.

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This change is 

Fixes #3985

[pullrequest]

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[pullrequest]

Thank you for this submission.

The code currently in place implements default server and mux are not suitable for this use case. I believe it creates a vector for users to obtain information about the profiled code that could be exploited further. I think this code can be improved by creating a server manually.

Consider this example, which achieve, I believe, the same outcome:

        addr := ":" + strconv.Itoa(port)
	ln, err := net.Listen("tcp", addr)

	server := http.Server{
		Addr:           addr,
		ReadTimeout:    5 * time.Second,
		WriteTimeout:   10 * time.Second,
		Handler:        nil,
		MaxHeaderBytes: 1024 * 1024,
	}

   go func() {
     if err := server.Serve(ln); err != nil {
        glog.Errorf("Error while starting HTTP server: %+v", err)
     }
   }()

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[pullrequest]

I think this approach can be refactored, and expose the components of the method for greater flexibility, particularly in environments that do not use the default mux but wish to participate in this feature.

[danielmai]

I believe it creates a vector for users to obtain information about the profiled code that could be exploited further.

Can you elaborate about this point? I'm not sure how exposing pprof with the default server and mux could cause an exploit.

[pullrequest]

order fields alphabetically.

[pullrequest]

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[gitlw]

Reviewable status: 0 of 1 files reviewed, 2 unresolved discussions (waiting on @danielmai, @manishrjain, and @pullrequest[bot])