refactor: move bytecounter to internal (#391)

It's generic enough to live outside of engine/netx.

Occurred to me while working on https://github.com/ooni/probe/issues/1687.

internal/bytecounter/bytecounter_test.go

@@ -0,0 +1,23 @@
+package bytecounter
+
+import "testing"
+
+func TestGood(t *testing.T) {
+	counter := New()
+	counter.CountBytesReceived(16384)
+	counter.CountKibiBytesReceived(10)
+	counter.CountBytesSent(2048)
+	counter.CountKibiBytesSent(10)
+	if counter.BytesSent() != 12288 {
+		t.Fatal("invalid bytes sent")
+	}
+	if counter.BytesReceived() != 26624 {
+		t.Fatal("invalid bytes received")
+	}
+	if v := counter.KibiBytesSent(); v < 11.9 || v > 12.1 {
+		t.Fatal("invalid kibibytes sent")
+	}
+	if v := counter.KibiBytesReceived(); v < 25.9 || v > 26.1 {
+		t.Fatal("invalid kibibytes received")
+	}
+}