Refactor SelectStatement

The current state of the code of SelectStatement make fixing some issues or adding new functionnalities really hard. It also contains some functionnalities that we would like to reuse in ModificationStatement but cannot for the moment.

Ideally I would like to:

• Perform as much validation as possible on Relations instead of performing it on Restrictions as it will help for problem like the one of #CASSANDRA-6075 (I believe that by clearly separating validation and Restrictions building we will also make the code a lot clearer)
• Provide a way to easily merge restrictions on the same columns as needed for #CASSANDRA-7016
• Have a preparation logic (validation + pre-processing) that we can easily reuse for Delete statement #CASSANDRA-6237
• Make the code much easier to read and safer to modify.