Dispose Virtual States option only via HippoSession#disposeVirtualStates() in the session pool when content change events triggered

This issue is a follow up het HSTTWO-1976 : There, Woonsan has abstracted away the session refresher to be pluggable in its behavior.

After REPO-305 has been addressed, we can continue with this issue

Ard recommeded this:

> If we want to gain some real 'short-term' improvements without having
> to wait for fundamental model changes, what do you think if we try to
> get the HippoSession method disposeVirtualStates() into the Hippo
> repository. Then, I think we can do with quite some less Session
> refreshes
>
> HippoSession extends Session

{ > void disposeVirtualStates() throws RepositoryException; > }

>
> This way, all our 'read-only' jcr sessions in the default session pool
> never need a refresh, but just a disposeVirtualStates() call when
> there was an event below /content caught
>

I think the BasicPoolingRepository#SessionFactory#activateObject() could
do it optionally (this can be the default configuration through
SpringComponentManager.properties by the way). The proxy session could
implement #disposeVirtualStates() to enable this.