perf(document+schema): small optimizations to make init() faster

[vkarpov15]

Re: #14113

Summary

Without this change:

$ node gh-14113.js 
6.12.6
mongoose spendTime: 24358 average: 48.716
$ node gh-14113.js 
6.12.6
mongoose spendTime: 24956 average: 49.912
$ node gh-14113.js 
6.12.6
mongoose spendTime: 25182 average: 50.364

With this change:

$ node gh-14113.js 
6.12.6
mongoose spendTime: 24003 average: 48.006
$ node gh-14113.js 
6.12.6
mongoose spendTime: 24222 average: 48.444
$ node gh-14113.js 
6.12.6
mongoose spendTime: 24247 average: 48.494

Seems like a small difference, but if you remove init entirely the average goes down to about 47.5, so this is actually a fairly significant speedup of init(). I'm assuming that the rest of the relatively small performance degradation in Mongoose 5 and 6 is due to bson slowdowns discussed in #13456.

Examples

[hasezoey]

Looks ok, but i will hold-off on approving until the main 6.x branch does not have failing tests anymore