Wrap studio views in bulk operations to minimize mongo calls.

@cpennington @nasthagiri @dmitchell 

There's more testing left to do - but the course index and unit pages are all set in my load tests.

Please pay particular attention to the split.py change. I removed a hard-coded param, which seemed incorrect.
Originally, depth was unimportant in split. It may be more important now. (In old mongo, it prefetches to the given depth, but since in split the whole tree is in the doc, prefetching is not important.) However, your change is reasonable b/c the old ignoring of depth was confusing (I probably didn't have depth as an arg before).