pgstat: stats collector references in comments.

Soon the stats collector will be no more, with statistics instead getting
stored in shared memory. There are a lot of references to the stats collector
in comments. This commit replaces most of these references with "cumulative
statistics system", with the remaining ones getting replaced as part of
subsequent commits.

This is done separately from the - quite large - shared memory statistics
patch to make review easier.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de
Discussion: https://postgr.es/m/20220308205351.2xcn6k4x5yivcxyd@alap3.anarazel.de

src/include/utils/backend_status.h

@@ -85,8 +85,8 @@ typedef struct PgBackendGSSStatus
  *
  * Each live backend maintains a PgBackendStatus struct in shared memory
  * showing its current activity.  (The structs are allocated according to
- * BackendId, but that is not critical.)  Note that the collector process
- * has no involvement in, or even access to, these structs.
+ * BackendId, but that is not critical.)  Note that this is unrelated to the
+ * cumulative stats system (i.e. pgstat.c et al).
  *
  * Each auxiliary process also maintains a PgBackendStatus struct in shared
  * memory.

src/include/utils/pgstat_internal.h

@@ -1,7 +1,7 @@
 /* ----------
  * pgstat_internal.h
  *
- * Definitions for the PostgreSQL activity statistics facility that should
+ * Definitions for the PostgreSQL cumulative statistics system that should
  * only be needed by files implementing statistics support (rather than ones
  * reporting / querying stats).
  *