Re-implement shell's DBCommandCursor in C++

The shell uses DBCommandCursor to iterate results returned from the "cursor-generating commands" (find, aggregate, listIndexes, listCollections). The implementation is primarily in Javascript:

https://github.com/mongodb/mongo/blob/bd76599a88e8062c634eeb6918bb30f868bc6042/src/mongo/shell/query.js#L650-L804

Rewriting this functionality in C++ would most likely make this code run faster. It would also ensure that the shell does not consume excessive memory, since memory required to buffer each batch of results could be reclaimed after the batch is exhausted. Currently, the Javascript engine's garbage collector may not run frequently enough to reclaim memory in a timely manner, especially for queries returning many megabytes worth of results.