Refactor BlockId into an actual type

This is an unfortunately invasive change which converts all of our BlockId
strings into actual BlockId types. Here are some advantages of doing this now:

+ Type safety

+ Code clarity - it's now obvious what the key of a shuffle or rdd block is,
  for instance. Additionally, appearing in tuple/map type signatures is a big
  readability bonus. A Seq[(String, BlockStatus)] is not very clear.
  Further, we can now use more Scala features, like matching on BlockId types.

+ Explicit usage - we can now formally tell where various BlockIds are being used
  (without doing string searches); this makes updating current BlockIds a much
  clearer process, and compiler-supported.
  (I'm looking at you, shuffle file consolidation.)

+ It will only get harder to make this change as time goes on.

Since this touches a lot of files, it'd be best to either get this patch
in quickly or throw it on the ground to avoid too many secondary merge conflicts.