Introduce populate ordered option for populating in series rather than in parallel

[vkarpov15]

Summary

Currently Mongoose executes all populate queries in parallel, which is problematic for transactions. Re: #15091, #15100, etc., MongoDB server does not support multiple operations in parallel within a single transaction. With this PR, you can populate with ordered: true as follows

Fix #15210

// Document.prototype.populate() with multiple paths
await docD
  .populate({
          path: ['refA', 'refB', 'refC'], 
          ordered: true
  })

// query.populate() with options
await Model.find().populate([{ path: 'refA', ordered: true }, { path: 'refB', ordered: true }]);

A couple of noteworthy caveats:

1. The options passed to populate() is always an array, because ordered is the only option we have that configures the top-level populate() call, as opposed to the individual _populatePaths() calls. That makes it somewhat tricky to pass ordered as a top-level option: assigning ordered as a property on the paths array is a bit error prone because it is easy to copy the array and lose ordered. So we made it so that we treat top-level populate as ordered if any of the populate paths are set to ordered, which seems like a reasonable workaround.
2. Before this PR, you couldn't pass an array of strings as populate path option. This PR allows passing an array of strings as path.

// This is fine in Mongoose 8.9, will populate `refA`, `refB`, `refC` as expected
await docD
  .populate({
          path: 'refA refB refC'
  })

// In Mongoose 8.9, the following would throw an error. With this PR, the following will be equivalent to the above
await docD
  .populate({
          path: ['refA', 'refB', 'refC'],
  })

This PR may be helpful for #13985, but I need to double check.

Examples

[hasezoey]

Code looks good, but there should be tests for this like other ordered operations, right?

[vkarpov15]

I would like to have tests for the populate ordered option, but testing is very tricky because we don't have a good way to ensure that the populate queries run in series rather than in parallel. Running queries in parallel in a transaction does not fail consistently. The only option I can think of right now is adding a $where to each populate to make each query slow, and assert that the full populate() is sufficiently slow, but that is slow and brittle.

[hasezoey]

I would like to have tests for the populate ordered option, but testing is very tricky because we don't have a good way to ensure that the populate queries run in series rather than in parallel

I can understand that it might be tricky to correctly test the behavior that this code path was made for, but with the way it currently is, it adds a diverging code path which, to my knowledge, is currently completely untested.
Personally i would just at least add some test that tests that this code path gives some correct result (and does not like run into a TypeError or something).

[vkarpov15]

That's fair, I added a test case in 5d2a5a4