Only synchonous version for now, since async C-API doesn't allow userdata

This requires support for out-of-isolate callbacks. To do this a dart
specific EventLoopDispatcher and DispatchFreeUserdata is introduced.

…eSyncConfiguration

Also, SubscriptionSet.remove* are now void methods, and corresponding
_RealmCore functions uses invokeGetBool.

When nothing is found we should return null on find. Hence we cannot use
invokeGetPointer, and also we need to pass null up the conversion stack.

* Add subscription tests to test_driver/realm_test.dart
* Allow network use from test_driver app on macos

The add method takes an optional update parameter, that default to
false. If true, the add will update the existing subscription if any.
Otherwise, it will fail if the subscription exists.

* remove is now called removeByQuery
* remove instead takes a Subscription as an argument
* removeAll is renamed to clear to match List.clear
* all remove* methods now returns a boolean indicating whether a
  subscription was removed

This is possible given recent changes to the C-API.

Using delay instead of awaiting sync session upload/download, since
SyncSession is not yet implemented.

…ate flag

* realm_dart_sync_session_wait_for_download_completion
* realm_dart_sync_session_wait_for_upload_completion
* realm_dart_sync_session_register_progress_notifier
* realm_dart_sync_session_register_connection_state_change_callback

* realm_dart_sync_session_wait_for_download_completion
* realm_dart_sync_session_wait_for_upload_completion
* realm_dart_sync_session_register_progress_notifier
* realm_dart_sync_session_register_connection_state_change_callback

* realm_dart_sync_session_wait_for_download_completion
* realm_dart_sync_session_wait_for_upload_completion
* realm_dart_sync_session_register_progress_notifier
* realm_dart_sync_session_register_connection_state_change_callback