The sharded insert code now correctly groups bulk inserts for different chunks, but the message size limit is larger than the bson size limit when retrying the bulk inserts via a WBL BSON object.

We can limit the bulk insert size we send to mongod to 1/2 the max BSON object size, or some other factor, however there's still the edge case when inserting documents very close to 16MB. We can still overrun the actual buffer size in this case due to the extra WBL metadata.