Remove CachingPage

This pull request removes the CachingPage from the WikiPage hierarchy. CachingPage is a bit of an odd thing. Also it prevented pages from working in a consistent, transactional way.

With this thing out of the way, the code has become more clear.
As a result pages can no longer be compared on identity (==), but have to be compared by value (Object.equals()).

Also ProxyPage is completely removed (a left-over from VirtualWiki).