pgsql: Fix planner crash from pfree'ing a partial path that a GatherPat

Fix planner crash from pfree'ing a partial path that a GatherPath uses.

We mustn't run generate_gather_paths() during add_paths_to_joinrel(),
because that function can be invoked multiple times for the same target
joinrel.  Not only is it wasteful to build GatherPaths repeatedly, but
a later add_partial_path() could delete the partial path that a previously
created GatherPath depends on.  Instead establish the convention that we
do generate_gather_paths() for a rel only just before set_cheapest().

The code was accidentally not broken for baserels, because as of today there
never is more than one partial path for a baserel.  But that assumption
obviously has a pretty short half-life, so move the generate_gather_paths()
calls for those cases as well.

Also add some generic comments explaining how and why this all works.

Per fuzz testing by Andreas Seltenreich.

Report: <871t5pgwdt.fsf@credativ.de>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c45bf5751b6338488bd79ce777210285531da373

Modified Files
--------------
src/backend/optimizer/README           | 15 ++++++++--
src/backend/optimizer/geqo/geqo_eval.c |  3 ++
src/backend/optimizer/path/allpaths.c  | 50 ++++++++++++++++++++--------------
src/backend/optimizer/path/joinpath.c  |  7 +----
src/backend/optimizer/util/pathnode.c  | 30 ++++++++++++++++----
5 files changed, 71 insertions(+), 34 deletions(-)