pgsql: Provide a planner hook at a suitable place for creating upper-re

Provide a planner hook at a suitable place for creating upper-rel Paths.

In the initial revision of the upper-planner pathification work, the only
available way for an FDW or custom-scan provider to inject Paths
representing post-scan-join processing was to insert them during scan-level
GetForeignPaths or similar processing. While that's not impossible, it'd
require quite a lot of duplicative processing to look forward and see if
the extension would be capable of implementing the whole query. To improve
matters for custom-scan providers, provide a hook function at the point
where the core code is about to start filling in upperrel Paths. At this
point Paths are available for the whole scan/join tree, which should reduce
the amount of redundant effort considerably.

(An alternative design that was suggested was to provide a separate hook
for each post-scan-join processing step, but that seems messy and not
clearly more useful.)

Following our time-honored tradition, there's no documentation for this
hook outside the source code.

As-is, this hook is only meant for custom scan providers, which we can't
assume very much about. A followon patch will implement an FDW callback
to let FDWs do the same thing in a somewhat more structured fashion.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5864d6a4b62ada2ad60a8c456b4ee62972a9c10d

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 26 ++++++++++++++++++++++++++
src/backend/optimizer/prep/prepjointree.c | 1 +
src/include/nodes/relation.h | 3 +++
src/include/optimizer/planner.h | 5 +++++
4 files changed, 35 insertions(+)