MRU cache implementation and fix for cache invalidation in ServletHandler

Jetty just has been dropping all cached filters when cache side exceeded, what was affecting on performance in large projects. To fix that ServletHandler._chainCache was reimpelemented using MRU cache.

In the process of redeploying/restarting of war bundle in OSGi environment any request to servlet can broke internal cache (MruCache._chainCache). This often happens with event polling mechanism of Ajax frameworks like ZK and Vaadin when client side beginning to receive 405 (Method POST is not supported) response instead of 200(OK).

Patch is attached.