Overhaul select DSL for 4.0

This ticket is to track many smaller changes which will cumulate into an overhauled and compatible select DSL for 4.0

• join only accept String, which is problematic for joins on buckets like "beer-sample" (should be escaped).
• in a join, the key clause is "KEYS" instead of "ON KEYS"
• for joins, it can be useful to be able to construct the pk by concatenation ("to_string("prefix_" || doc.foreignKey)"), so maybe provide to_string in Functions