Add Profiler Admin API (#6463)

Two handlers are added to admin API to enable profiling and disable
profiling of a server in a standalone mode, or all nodes in the
distributed mode.

/minio/admin/profiling/start/{cpu,block,mem}:
  - Start profiling and return starting JSON results, e.g. one
    node is offline.

/minio/admin/profiling/download:
  - Stop the on-going profiling task
  - Stream a zip file which contains all profiling files that can
    be later inspected by go tool pprof

cmd/common-main.go

@@ -98,7 +98,9 @@ func handleCommonCmdArgs(ctx *cli.Context) {
 func handleCommonEnvVars() {
 	// Start profiler if env is set.
 	if profiler := os.Getenv("_MINIO_PROFILER"); profiler != "" {
-		globalProfiler = startProfiler(profiler)
+		var err error
+		globalProfiler, err = startProfiler(profiler, "")
+		logger.FatalIf(err, "Unable to setup a profiler")
 	}
 
 	accessKey := os.Getenv("MINIO_ACCESS_KEY")