Move all status routes to use the mux router

[yzhan1]

Enhancement

Currently, some routes in the status endpoint are registered to the http.ServeMux directly, while some are registered to the mux.Router. Since both serves the same purpose and there seems to be no custom handling between routes registered behind ServeMux and Router, it's better to consolidate them so we only register routes on the mux.Router. It's cleaner and makes it easier in the future if we want to add a middleware for all routes.

Sample:

tidb/pkg/server/http_status.go

Line 300 in f4e366e

serverMux.HandleFunc("/debug/pprof/", pprof.Index)