Disable logging threads when running in release mode

[swift-kim]

Dart messages should be logged through the new logging callback recently added to the embedder APIs as suggested by #105.

The current plan:

• When running in release mode, only Dart messages are logged to dlog.
• When running in debug mode, Dart messages and all stdout/stderr messages are logged to dlog.

[swift-kim]

Unhandled exception reports are not treated as Dart messages, but instead they are just printed out to the standard output through fml. We can't apply the above approach without modifying fml itself because we still need to redirect engine messages (such as unhandled exception reports) to dlog.

[swift-kim]

There's no good solution for this issue.

See #129 (comment).