pgsql: Add MODE option to CHECKPOINT command.

Add MODE option to CHECKPOINT command.

This option may be set to FAST (the default) to request the
checkpoint be completed as fast as possible, or SPREAD to request
the checkpoint be spread over a longer interval (based on the
checkpoint-related configuration parameters). Note that the server
may consolidate the options for concurrently requested checkpoints.
For example, if one session requests a "fast" checkpoint and
another requests a "spread" checkpoint, the server may perform one
"fast" checkpoint.

Author: Christoph Berg <myon(at)debian(dot)org>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Reviewed-by: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
Reviewed-by: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2f698d7f4b7b4c49c3649b2fcc063eb66f9d2e6c

Modified Files
--------------
doc/src/sgml/ref/checkpoint.sgml | 35 ++++++++++++++++++++++++++++++++---
src/backend/postmaster/checkpointer.c | 28 +++++++++++++++++++++++-----
src/bin/psql/tab-complete.in.c | 13 +++++++++++++
src/test/regress/expected/stats.out | 9 +++++++--
src/test/regress/sql/stats.sql | 6 ++++--
5 files changed, 79 insertions(+), 12 deletions(-)