The way ORCA was tied into the planner, running a planner_hook
was not supported in the intended way. This commit moves ORCA
into standard_planner() instead of planner() and leaves the hook
for extensions to make use of, with or without ORCA. Since the
intention with the optimizer GUC is to replace the planner in
postgres, while keeping the planning proess, this allows for
planner extensions to co-operate with that.

In order to reduce the Greenplum footprint in upstream postgres
source files for future merges, the ORCA functions are moved to
their own file.

Also adds a memaccounting class for planner hooks since they
otherwise ran in the planner scope, as well as a test for using
planner_hooks.