Rework handling of pending data for backend statistics

Rework handling of pending data for backend statistics

9aea73fc61d4 has added support for backend statistics, relying on
PgStat_EntryRef->pending for its data pending for flush.  This design
lacks in flexibility, because the pending list does some memory
allocation, making it unsuitable if incrementing counters in critical
sections.

Pending data of backend statistics is reworked so the implementation
does not depend on PgStat_EntryRef->pending anymore, relying on a static
area of memory to store the counters that are flushed when stats are
reported to the pgstats dshash.  An advantage of this approach is to
allow the pending data to be manipulated in critical sections; some
patches are under discussion and require that.

The pending data is tracked by PendingBackendStats, local to
pgstat_backend.c.  Two routines are introduced to allow IO statistics to
update the backend-side counters.  have_static_pending_cb and
flush_static_cb are used for the flush, instead of flush_pending_cb.

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