pg_stat_wal: Accumulate time as instr_time instead of microseconds

pg_stat_wal: Accumulate time as instr_time instead of microseconds

In instr_time.h it is stated that:

* When summing multiple measurements, it's recommended to leave the
* running sum in instr_time form (ie, use INSTR_TIME_ADD or
* INSTR_TIME_ACCUM_DIFF) and convert to a result format only at the end.

The reason for that is that converting to microseconds is not cheap, and can
loose precision.  Therefore this commit changes 'PendingWalStats' to use
'instr_time' instead of 'PgStat_Counter' while accumulating 'wal_write_time'
and 'wal_sync_time'.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/1feedb83-7aa9-cb4b-5086-598349d3f555@gmail.com