pgsql: Use condition variables to wait for checkpoints.

Use condition variables to wait for checkpoints.

Previously we used a polling/sleeping loop to wait for checkpoints
to begin and end, which leads to up to a couple hundred milliseconds
of needless thumb-twiddling.  Use condition variables instead.

Author: Thomas Munro
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/CA%2BhUKGLY7sDe%2Bbg1K%3DbnEzOofGoo4bJHYh9%2BcDCXJepb6DQmLw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c6c9474aafa4de357ae424cd18b69e8bf7a4babe

Modified Files
--------------
doc/src/sgml/monitoring.sgml          | 10 +++++++++-
src/backend/postmaster/checkpointer.c | 21 +++++++++++++++++----
src/backend/postmaster/pgstat.c       |  6 ++++++
src/include/pgstat.h                  |  2 ++
4 files changed, 34 insertions(+), 5 deletions(-)