Simplify NodeFinder with a lambda.
Remove GenericFileFilter and GenericFileFilters.

Use a plain Predicate<GenericFile> for the former, and define methods on
GenericFile for the latter (can use method references!).
Simplify MergeUtil.

In Java 8 there's a new merge method on Map, which can replace
MergeUtil.mergePut. We can also use lambdas instead of creating mergers.
Some more Java8 powered simplifications.