AsyncContext: using existing transactional context

[juliano]

Problem

Same as described in quill, AsyncContext doesn't handle nested transactions correctly

Solution

Use the existing TransactionalExecutionContext

Notes

Additional notes.

Checklist

• Unit test all changes
• Update README.md if applicable
• Add [WIP] to the pull request title if it's work in progress
• Squash commits that aren't meaningful changes
• Run sbt scalariformFormat test:scalariformFormat to make sure that the source files are formatted

@getquill/maintainers