perf: memoize toJSON / toObject default options

[vkarpov15]

Summary

Re: #14394, we can get a maybe 2.5% speedup by not combining Mongoose global options and schema base options every time we call $toObject(). Not a huge speedup, but every little bit helps given that $toObject() is called on some heavily used code paths, like save() and JSON.stringify().

I'll put this change in 8.5, just to de-risk because it may cause some issues for people who modify toJSON and toObject options after compiling their schemas.

Examples