use `deleteMany` for db cleaning

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DB cleaning as currently implemented could result in high network activity during cleanup. Use mongo db delete command instead

Current implementation

• https://github.com/roshub/vapor-master/blob/master/src/master.js#L33
• https://github.com/roshub/vapor-master/blob/master/src/core-util.js#L9
• https://github.com/roshub/vapor-master/blob/master/src/service-util.js#L7

https://mongoosejs.com/docs/api.html#model_Model.deleteMany

https://docs.mongodb.com/manual/reference/method/db.collection.deleteMany/