refactor: move bytecounting conn in bytecounter pkg (#392)

* refactor: move bytecounting conn in bytecounter pkg

This enables other pieces of code to request bytecounting without
depending on netx or on the perverse using-the-context-to-configure-
byte-counting mechanism.

Also occurred when working on https://github.com/ooni/probe/issues/1687

* fix: add missing docs

internal/engine/netx/dialer/bytecounter_test.go

@@ -48,9 +48,15 @@ func TestByteCounterNormalUsage(t *testing.T) {
 	if err := dorequest(ctx, "http://facebook.com"); err != nil {
 		t.Fatal(err)
 	}
+	if exp.Received.Load() <= 0 {
+		t.Fatal("experiment should have received some bytes")
+	}
 	if sess.Received.Load() <= exp.Received.Load() {
 		t.Fatal("session should have received more than experiment")
 	}
+	if exp.Sent.Load() <= 0 {
+		t.Fatal("experiment should have sent some bytes")
+	}
 	if sess.Sent.Load() <= exp.Sent.Load() {
 		t.Fatal("session should have sent more than experiment")
 	}