feat: optimise and test code for various versions of MongoDB i.e. v3.…

…4, v3.6, v4.0, v4.2

Code base optimized and tested against

MongoDB v3.4, v3.6, v4.0, v4.2
Node version v8, v10 and v12

There was an issue why publishing from previous pull #2 . Redo to ensure correct semver numbers set

README.md

@@ -250,7 +250,7 @@ console.log(droid1.make) ---> '18Y4434'
 It may arise that you need to query and use the nextId to at the front-end. In this case, you just need to 
 get an instance of the Assigner, then use the `getNextId` method. It is async method as it queries for `Number` and `String` cases.
 
-### Strain test
+## Strain test
 * Performs the task below on a locally hosted db instance.
 * On CI/CD environment, tests ran using [mongodb-memory-server](https://github.com/nodkz/mongodb-memory-server), on v3.4, v3.6, v4.0 and v4.2 on node v12, v10, v8