Performance improvements for insertMany()

[vkarpov15]

Summary

The big changes are:

1. a _skipParallelValidateCheck option for validate(). Looks like creating a new ParallelValidateError() each time is relatively expensive, and unnecessary for insertMany() since the user won't have access to the document references anyway outside of middleware.
2. Skip $toObject() in favor of $__toObjectInternal(), which skips options processing and only does a shallow clone, when using insertMany() on documents which only have primitive values (no subdocs, nested paths, or arrays).

Before this PR:

$ node gh-14719.js 
{
  "Average insertMany time ms": 54.99
}

After this PR:

$ node gh-14719.js 
{
  "Average insertMany time ms": 20.66
}

Looking at about 2.5x performance improvement.

Potential alternative would be to change this.$__.validating to use a boolean instead of an error instance, will investigate.

Examples

[hasezoey]

Looks good to me, one minor suggestion