Before this commit, BlockStructures were being cached with one
entry for the top level data (structure, course-wide transform
data), and an entry for each block's data. There can be thousands
of blocks in a course. While this makes it so that you can grab
a small subset of blocks without incurring much overhead,
splitting the block information means that the overall size of the
data stored for generating course-level views increases drastically,
since compression is less efficient.
Making this change yields about a 7X decrease in serialized data
size.
