fix(populate): split into separate populate per document if resulting $in has more than 50k elements to reduce risk of BSON errors

[vkarpov15]

Fix #5890

Summary

The crux of the issue in #5890 is that populate() can lead to BSON size limit errors in the case of multiple documents. About 800k ObjectIds fit in the BSON size limit - however, populate() on multiple documents can try to stuff more ids than that into a query. For example, if we have a relationship User -> TVShow -> View, the populate() call for View can include the views from multiple TVShow documents, and lead to BSON overflow.

The practical solution I came up with is to switch to perDocumentLimit behavior (one query per document vs per model) if the resulting query has more than 50k elements. That limit is intentionally not very rigorous, because calculating the BSON serialized size for a large array can be performance intensive; but the 50k limit is enough to avoid BSON overflows for ObjectIds, uuids, dates, and numbers.

The key insight behind splitting by document is that documents themselves must fit within the BSON size limit, so an individual document's populate() cannot trigger a BSON size limit presuming that document was already saved in MongoDB (of course possible to create a document in memory with an ObjectId array of length 1M and try to populate it but not very realistic because you could never save this document).

Examples

[hasezoey]

LGTM