refactor / repo structure and style changes

[yzhang1994]

At @mifeng's request, I looked at the Gateway API repo. The goal is to make sure we are following best practices.

Here is a list of changes I'm proposing. I also implemented several changes as a part of this PR:

• Re-organize repo structure so that it is more standard to nodeJS conventions. Please refer to this guide for the rationale behind the re-org. The re-org is already done as a part of this PR.

• Add a code linter, which is standard for JS repos. This is included in the PR as well.

• Separated app routing logic from business logic. This is recommended because it makes the codebase more readable, and allows developers to test individual components. I refactored the balancer route as an example.

• Use modern JS ES6. Meaning import, export, class, etc. This has less direct impact, but is also pretty standard. I refactored relevant code in the balancer route as examples.

• Specify node/npm/yarn version in package.json. I didn't do this because I'm not sure what version of node @sdgoh is using currently. But this is important because it could cause version incompatibility later on, and is always a pain to debug.

• Long term: add unit tests.

• Long term: consider converting to Typescript. For a backend API, typing will help improve stability.

[fengtality]

I'll resolve merge conflicts and merge this in shortly

[gyozajiro]

Looks good. Thanks for refactoring it.