Test/Enable Node.js 12 for noSQL connectors

[dhmlau]

Description / Steps to reproduce / Feature proposal

Originated from: #3072

reference -> MySQL

Acceptance Criteria

For the following noSQL connectors, test locally on Node.js 12 and see if all tests passed.

If yes, add Node.js 12 to CI
If not, try to fix it for simple fixes. If you don't think you can fix it in a day, open a new issue to keep track of it.

• loopback-connector-mongodb -> Add Node.js 12 to Travis CI + fix failing tests loopback-connector-mongodb#518
• loopback-connector-couchdb2 -> drop support for node 6 loopback-connector-couchdb2#67
• loopback-connector-cloudant -> chore: add node 12 to travis ci loopback-connector-cloudant#212
• loopback-connector-cassandra -> Drop support for Node.js 6 and add Node.js 12 to CI loopback-connector-cassandra#78

[nabdelgadir]

Closing as done.