pgsql: Include information on buffer usage during planning phase, in EX

Include information on buffer usage during planning phase, in EXPLAIN output.

When BUFFERS option is enabled, EXPLAIN command includes the information
on buffer usage during each plan node, in its output. In addition to that,
this commit makes EXPLAIN command include also the information on
buffer usage during planning phase, in its output. This feature makes it
easier to discern the cases where lots of buffer access happen during
planning.

Author: Julien Rouhaud, slightly revised by Fujii Masao
Reviewed-by: Justin Pryzby
Discussion: https://postgr.es/m/16109-26a1a88651e90608@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ed7a5095716ee498ecc406e1b8d5ab92c7662d10

Modified Files
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src/backend/commands/explain.c        | 32 ++++++++++++++++++--
src/backend/commands/prepare.c        | 13 ++++++++-
src/include/commands/explain.h        |  3 +-
src/test/regress/expected/explain.out | 55 ++++++++++++++++++++++++++++++++---
4 files changed, 95 insertions(+), 8 deletions(-)