pgsql: Rename some pgstats callbacks related to flush of entries

Rename some pgstats callbacks related to flush of entries

The two callbacks have_fixed_pending_cb and flush_fixed_cb have been
introduced in fc415edf8ca8 to provide a way for fixed-numbered
statistics to control the flush of their data. These are renamed to
respectively have_static_pending_cb and flush_static_cb. The
restriction that these only apply to fixed-numbered stats is removed.

A follow-up patch will make use of them for backend statistics. This
stats kind is variable-numbered, and patches are under discussion to
track WAL data for IO and backend stats which cannot use
PgStat_EntryRef->pending as pending data would be touched in critical
sections, where no memory allocation can happen.

Per discussion with Andres Freund.

Author: Bertrand Drouvot
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/66efowskppsns35v5u2m7k4sdnl7yoz5bo64tdjwq7r5lhplrz@y7dme5xwh2r5

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/28de66cee5f45e2f173fa60dd6867c810ecabe38

Modified Files
--------------
src/backend/utils/activity/pgstat.c | 36 +++++++++++++-----------------------
src/include/utils/pgstat_internal.h | 27 +++++++++++++++++----------
2 files changed, 30 insertions(+), 33 deletions(-)