Flush feature source command

[jng]

A common problem with a Feature Source that is in constant flux is that schema information is cached for performance reasons resulting in the schema information being out of date.

To flush this cached item normally you would need to call SaveResource on the resource id of that feature source, which triggers invalidation of that cached item.

To make this process dead simple, add a new "Flush Feature Source from cache" command. The implementation of this command is also simple. We take the resource content from a resource id and write it back to the same id. This will trigger cache invalidation, ensuring the next schema walk operation gives the current information.

This would not affect the any open editors of the same resource, as they are operating off of a session-based copy.

[jng]

Fixed in r5670