pgsql: Split backend status and progress related functionality out of p

Split backend status and progress related functionality out of pgstat.c.

Backend status (supporting pg_stat_activity) and command
progress (supporting pg_stat_progress*) related code is largely
independent from the rest of pgstat.[ch] (supporting views like
pg_stat_all_tables that accumulate data over time). See also
a333476b925.

This commit doesn't rename the function names to make the distinction
from the rest of pgstat_ clearer - that'd be more invasive and not
clearly beneficial. If we were to decide to do such a rename at some
point, it's better done separately from moving the code as well.

Robert's review was of an earlier version.

Reviewed-By: Robert Haas <robertmhaas(at)gmail(dot)com>
Discussion: https://postgr.es/m/20210316195440.twxmlov24rr2nxrg@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e1025044cd4e7f33f7304aed54d5778b8a82cd5d

Modified Files
--------------
src/backend/bootstrap/bootstrap.c | 5 +-
src/backend/postmaster/pgstat.c | 1125 +------------------------
src/backend/utils/activity/Makefile | 2 +
src/backend/utils/activity/backend_progress.c | 112 +++
src/backend/utils/activity/backend_status.c | 1077 +++++++++++++++++++++++
src/backend/utils/init/postinit.c | 4 +
src/backend/utils/misc/guc.c | 1 +
src/include/commands/progress.h | 2 +-
src/include/pgstat.h | 302 +------
src/include/utils/backend_progress.h | 44 +
src/include/utils/backend_status.h | 316 +++++++
11 files changed, 1598 insertions(+), 1392 deletions(-)