Update piece handling

[bedeho]

Background

Prior, we had internal piece picking in the session, which are are no longer doing. As such, we had a very rich representation of pieces inside the session (detail::Piece) and also required availability information upon starting (PieceInformation).

Problem

• We have moved away from this, and the work on the piece picking callback revealed that this needs to be changed. In particular, too much information is leaking through that callback, and the session itself is still requiring information it doesn´t need (e.g. PieceInformation).
• We have not updated the tests to work with this new external piece picking

Solution

Fix both, in particular, make sure to

• kill PieceInformation
• don´t leak useless information in the piece picking callback
• update tests