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