Document.onRelease() should be private?

[hajimetakase]

I think if Document.onRelease() is executed before the commit(), then the data could be corrupted(because it does incrementValue = null;

flamingo/flamingo/lib/src/model/increment.dart

Line 41 in 7437db4

incrementValue = null;

).
So maybe Document.onRelease() should be private?

Also, I sometime wants to access to Document.value after the increment, so maybe there should be some field like Document.promisedValue which return the value after the commit() beforehand.

int get promisedValue => { 
if (incrementValue != null) {
        final _value =
            value != null ? value! + incrementValue! : incrementValue;
        result = Increment<T>(value: _value! as T);
      } else {
        result = Increment<T>(value: value);
      }
return result.value
}