Put the unique path in the same context as the owning RelOptInfo, rather
than the toplevel planner context.  This is how this function worked
originally, but commit f41803bb
changed it without explanation.  mark_dummy_rel adopted the older (or
newer?) technique in commit eca75a12,
which also featured a much better explanation of why it is correct.
So, switch back to that technique here, with the same explanation
given there.

Although this fixes a possible memory leak when GEQO is in use, the
leak is minor and probably nobody cares, so no back-patch.

Ashutosh Bapat, reviewed by Tom Lane and by me

Discussion: http://postgr.es/m/CAFjFpRcXkHHrXyD9BCvkgGJV4TnHG2SWJ0PhJfrDu3NAcQvh7g@mail.gmail.com