Underflow possible in `PropertyValueCountTrigger` implementation

[RobertJacobsonCDC]

This is a bug in the current implementation of the new PropertyValueCountTrigger in the new triggers module. See #991 for the fundamental cause; see #990 for a working demonstration of the failure.

Unfortunately, there doesn't appear to be a way to implement a simple property value count in client code outside of addressing #991. So in order to use this trigger we need to have some implementation outside of the triggers module in Ixa core, presumably in PropertyValueStoreCore<E, P>.

Some possibilities:

1. Require any property you use with PropertyValueCountTrigger to be IndexableProperty and use ValueCountIndex to implement. Not great because it counts all values, not just the value of interest. Ok when number of distinct values is small.
2. Require any property you use with PropertyValueCountTrigger to be IndexableProperty and implement a new single-value counter index. Properties could have many single value indexes, just like they can have many ValueChangeCountIndex instances (e.g. for daily, weekly incidence tracking).
3. Implement a single-value counter index that doesn't require the property to be IndexableProperty. This is tricky: you'd need to search for the count of the property value of index, which is fine if you have <~10 values of interest but degrades quickly.

[RobertJacobsonCDC]

I think the right path forward in the short term is to implement PropertyInitializedEvent. So let's block this temporarily until we can change the implementation to be based on that feature.

Note this means until we fix it, PropertyValueCountTrigger can trigger a panic. Client code prototyping with the new triggersmodule will just need to be careful how they structure their code until then.