This removes some code:

- The clique engine calculated the snapshot twice when verifying headers/blocks.

- The method GetBlockHashesFromHash in Header/Block/Lightchain was only used by tests. It
  is now removed from the API.
  
- The method GetTdByHash internally looked up the number before calling GetTd(hash, num).
  In many cases, callers already had the number, and used this method just because it has a
  shorter name. I have removed the method to make the API surface smaller.