Remove PlannerInfo's join_search_private method.

Remove PlannerInfo's join_search_private method.

Instead, use the new mechanism that allows planner extensions to store
private state inside a PlannerInfo, treating GEQO as an in-core planner
extension.  This is a useful test of the new facility, and also buys
back a few bytes of storage.

To make this work, we must remove innerrel_is_unique_ext's hack of
testing whether join_search_private is set as a proxy for whether
the join search might be retried. Add a flag that extensions can
use to explicitly signal their intentions instead.