FreeBlockDictionary is an interface for BinaryTreeDictionary with no other implementations. Having a template superclass for BinaryTreeDictionary just complicates the code for no good reason.
There is only one location where a base class pointer to a FreeBlockDictionary is used, in CompactibleFreeListSpace, but that can easily be replaced by the concrete type used all over CMS instead.
Additionally, the interface allows implementations to return chunks of roughly correct sizes specified by the Dither enum. No caller ever requests anything but the default atLeast selection so we should get rid of that parameter as well.