Add eslint-plugin-promise and fix async/Promise issues

[humphd]

We have a bunch of improper uses of async (e.g., without await) and also a bunch of Promise bugs that will affect performance.

A first step on the path to fixing this would be to add some linting for Promises. We can use https://github.com/xjamundx/eslint-plugin-promise, with the recommended rules defined. I tested it locally, and it already finds 24 problems we can fix.

We should also eliminate all pure synchronous calls that have been wrapped with Promises unnecessarily. In essence, if we don't need a Promise, let's get rid of it.

This recent talk is a good intro to some of the problems I'm referring to here.

[humphd]

Let's also add these rules:

• https://eslint.org/docs/rules/require-await
• https://eslint.org/docs/rules/no-async-promise-executor
• https://eslint.org/docs/rules/no-await-in-loop
• https://eslint.org/docs/rules/require-atomic-updates
• https://eslint.org/docs/rules/no-return-await