quill-jdbc: support nested transaction calls

[fwbrasil]

Fixes #402

Problem

The jdbc context doesn't support nested transactions because each transactional call creates a new transaction.

Solution

Don't create a new transaction if the transaction call is already inside a transaction

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

[jilen]

👍

[fwbrasil]

👍