Add support for mongodb driver v4

Issue Description

Exactly in this commit mongodb stopped exporting a Cursor object.

Because of that, every reference to ParentCursor.Prototype in MongoEntityManager (like here) is failing with error “Cannot read property ‘prototype’ of undefined”.

Expected Behavior

No error expected.

Actual Behavior

Error “Cannot read property ‘prototype’ of undefined”

TypeError: Cannot read property 'prototype' of undefined
    at FindCursor.cursor.toArray (/entity-manager/MongoEntityManager.ts:679:37)
    at MongoEntityManager.<anonymous> (/entity-manager/MongoEntityManager.ts:97:23)
    at step (/node_modules/tslib/tslib.js:143:27)
    at Object.next (/node_modules/tslib/tslib.js:124:57)
    at fulfilled (/node_modules/tslib/tslib.js:114:62)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)

Steps to Reproduce

1. Add typeorm dependency
2. Add mongodb v4 dependency.

My Environment

Dependency	Version
Operating System	Ubuntu 18.04 on wsl2
Node.js version	v14.17.0
Typescript version	v4.0.8
TypeORM version	v0.2.34

Additional Context

Relevant Database Driver(s)

• aurora-data-api
• aurora-data-api-pg
• better-sqlite3
• cockroachdb
• cordova
• expo
• mongodb
• mysql
• nativescript
• oracle
• postgres
• react-native
• sap
• sqlite
• sqlite-abstract
• sqljs
• sqlserver

Are you willing to resolve this issue by submitting a Pull Request?

• Yes, I have the time, and I know how to start.
• Yes, I have the time, but I don’t know how to start. I would need guidance.
• No, I don’t have the time, although I believe I could do it if I had the time…
• No, I don’t have the time and I wouldn’t even know how to start.

[mschnee]

🚨🚨🚨 Customers using certain tiers of MongoDB Atlas are being force-upgraded to MongoDB 5 this month as Mongo is dropping support for older databases. The minimum driver version supported is 4.0. https://docs.mongodb.com/drivers/node/current/compatibility/ 🚨🚨🚨

[imnotjames]

Currently, TypeORM is only compatible with mongo node v3.