Deduplicate parsing #608

This issue is an extract from #558 (comment)
We parse and build the AST several times. If we look at execute_with_reader():

• It builds the source tree, builds the AST for dispatch purposes and calls validate()
• validate() itself also rebuilds the source tree and the AST internally (for the 2nd time)
• execute_with_reader() passes the query on prepare_data_with_reader(), not the source tree / AST that were already build. It also does parsing and AST building (for the 3rd time).