Add accessor methods to Resource to make down-cast unnecessary

`AccessPoint` blindly down-casts `Resource` to `WarcResource` for accessing revisit info specific to WARCs.
It causes `ClassCastException` when `JWATFlexStore` is used for parsing WARC files (WWM-101).

This patch adds two new methods to `Resource` to let `AccessPoint` access revisit info without down-cast.

It also moves `AccessPoint.isWarcRevisitNotModified` method (also contains down-cast) to `WarcResource`. This method is not added to `Resource` class because it's apparently unused.