Scope driver to Mongoose instance

[vkarpov15]

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Issue

Right now, we only support one driver per Mongoose module. require('./lib/driver') is a singleton.

Ideally, we'd instead have driver as a property on a Mongoose instance. So a new Mongoose instance new mongoose.Mongoose() can have a different driver.

I suspect this will also help clean up some circular dependencies and other build issues (like #12335).

Something to consider for 7.0.