pgsql: Add ExplainState argument to pg_plan_query() and planner().

Add ExplainState argument to pg_plan_query() and planner().

This allows extensions to have access to any data they've stored
in the ExplainState during planning. Unfortunately, it won't help
with EXPLAIN EXECUTE is used, but since that case is less common,
this still seems like an improvement.

Since planner() has quite a few arguments now, also add some
documentation of those arguments and the return value.

Author: Robert Haas <rhaas@postgresql.org>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://postgr.es/m/CA+TgmoYWKHU2hKr62Toyzh-kTDEnMDeLw7gkOOnjL-TnOUq0kQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c83ac02ec7309edb7561eee93895c31a54b93d3d

Modified Files
--------------
contrib/pg_stat_statements/pg_stat_statements.c    | 14 ++++++-----
src/backend/commands/copyto.c                      |  2 +-
src/backend/commands/createas.c                    |  2 +-
src/backend/commands/explain.c                     |  2 +-
src/backend/commands/matview.c                     |  2 +-
src/backend/commands/portalcmds.c                  |  3 ++-
src/backend/optimizer/plan/planner.c               | 27 ++++++++++++++++++----
src/backend/tcop/postgres.c                        |  7 +++---
src/include/optimizer/optimizer.h                  |  5 +++-
src/include/optimizer/planner.h                    |  8 +++++--
src/include/tcop/tcopprot.h                        |  4 +++-
src/test/modules/delay_execution/delay_execution.c |  7 +++---
12 files changed, 58 insertions(+), 25 deletions(-)