Refactor: add a private constructor for connector

[dolmen]

Description

This is a minor refactoring that will allow to prepare more things when the Connector is created and do less on each Connect. Those changes will be proposed in further PRs (spoil: I intend to move some code from handleParams once #1111 is merged).

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[julienschmidt]

The change as it currently is, is a bit useless.
But #1111 was merged. Feel free to start moving code ;)

[dolmen]

I agree this minor refactoring is a "a bit useless", but at least it should not be controversial and is a good base for further changes (which might be more controversial and might require multiple tries that would all start with this change).

[julienschmidt]

And there is nothing wrong with including this very minor change in the maybe controversial change.
This PR is not an improvement.