make currentConnection protected

[schmitch]

Problem

Actually at the moment it's really hard to convert our JDBC code to quill due to the fact that we can't override/change the currentConnection.

At the moment we just want to create a Adapter that takes a implicit connection and does the same thing like transaction except getting a Connection via withConnection.

Solution

Sets currentConnection to protected.

Current way: override transaction and withConnection and using our own currentConnection.

Notes

none

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

use case: creating a transaction adapter between other jdbc libraries and quill

[fwbrasil]

@schmitch thanks! :)