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/utils_test.go

@@ -228,8 +228,9 @@ func TestURL2BucketObjectName(t *testing.T) {
 
 // Add tests for starting and stopping different profilers.
 func TestStartProfiler(t *testing.T) {
-	if startProfiler("") != nil {
-		t.Fatal("Expected nil, but non-nil value returned for invalid profiler.")
+	_, err := startProfiler("", "")
+	if err == nil {
+		t.Fatal("Expected a non nil error, but nil error returned for invalid profiler.")
 	}
 }