Add model, deleteModel events to connection

[vkarpov15]

Do you want to request a feature or report a bug?

Feature

What is the current behavior?

Right now there is no way for a plugin to be notified if a model is deleted. The 'init' event on schemas can tell you when a model is init-ed, but that is counterintuitive.

What is the expected behavior?

conn.on('model', model => console.log('Created model ' + model.modelName));
conn.on('deleteModel', model => console.log('Deleted model ' + model.modelName));

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Mongoose 5.11.x. Node.js and MongoDB versions don't matter for this issue.