refactor(NODE-6398): bulkWrite internals to use async/await

[nbbeeken]

Description

What is changing?

• Refactor executeCommands to use async/await

Is there new documentation needed for these changes?

No

What is the motivation for this change?

Stack traces, debugging

Double check the following

• [x] Ran npm run check:lint script
• [x] Self-review completed using the steps outlined here
• [x] PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• [x] Changes are covered by tests
• [x] New TODOs have a related JIRA ticket