The comments of CompactHashtable and related classes are out of date, and some comments are unclear, wrong or misleading.

As the related classes are used in more and more scenarios, it's helpful to update the comments.

To improve readability (of both the source code and the comments), we should also rename some of the field/parameters to distinguish between the type V (the "value " stored inside the table; it could be a 64-bit pointer) vs "encoded value" (the 32-bit representation of V in the CompactHashtable storage).