separate lock from common grid to avoid epoll contention (#20180)

epoll contention on TCP causes latency build-up when
we have high volume ingress. This PR is an attempt to
relieve this pressure.

upstream issue https://github.com/golang/go/issues/65064
It seems to be a deeper problem; haven't yet tried the fix
provide in this issue, but however this change without
changing the compiler helps. 

Of course, this is a workaround for now, hoping for a
more comprehensive fix from Go runtime.

cmd/generic-handlers_test.go

@@ -64,6 +64,14 @@ func TestGuessIsRPC(t *testing.T) {
 	if !guessIsRPCReq(r) {
 		t.Fatal("Grid RPC path not detected")
 	}
+	r = &http.Request{
+		Proto:  "HTTP/1.1",
+		Method: http.MethodGet,
+		URL:    &url.URL{Path: grid.RouteLockPath},
+	}
+	if !guessIsRPCReq(r) {
+		t.Fatal("Grid RPC path not detected")
+	}
 }
 
 var isHTTPHeaderSizeTooLargeTests = []struct {