SEARCH and CYCLE clauses

 SEARCH and CYCLE clauses
 
 This adds the SQL standard feature that adds the SEARCH and CYCLE
 clauses to recursive queries to be able to do produce breadth- or
 depth-first search orders and detect cycles.  These clauses can be
 rewritten into queries using existing syntax, and that is what this
 patch does in the rewriter.
 
 Reviewed-by: Vik Fearing <vik@postgresfriends.org>
 Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
 Discussion: https://www.postgresql.org/message-id/flat/db80ceee-6f97-9b4a-8ee8-3ba0c58e5be2@2ndquadrant.com