Fix for race condition in interface interceptors

Hi Mike,

I ran into an issue where the interface property interceptor classes were throwing errors due to a race condition during the setup of their respective 'value' dictionaries - we were hoping to be able to cache certain items for re-use across multiple page loads like you would regular Sitecore items.

Hopefully this fix will address that issue, but I would be interested to know your stance on the cachability (and thread-safety) of Glass items.

Thanks,
Chris