Re: Implement waiting for wal lsn replay: reloaded

Could this be causing the recent flapping failures on CI/macOS in
recovery/031_recovery_conflict?  I didn't have time to dig personally
but f30848cb looks relevant:

Waiting for replication conn standby's replay_lsn to pass 0/03467F58 on primary
error running SQL: 'psql:<stdin>:1: ERROR:  canceling statement due to
conflict with recovery
DETAIL:  User was or might have been using tablespace that must be dropped.'
while running 'psql --no-psqlrc --no-align --tuples-only --quiet
--dbname port=25195
host=/var/folders/g9/7rkt8rt1241bwwhd3_s8ndp40000gn/T/LqcCJnsueI
dbname='postgres' --file - --variable ON_ERROR_STOP=1' with sql 'WAIT
FOR LSN '0/03467F58' WITH (MODE 'standby_replay', timeout '180s',
no_throw);' at /Users/admin/pgsql/src/test/perl/PostgreSQL/Test/Cluster.pm
line 2300.

https://cirrus-ci.com/task/5771274900733952

The master branch in time-descending order, macOS tasks only:

     task_id      | substring |  status
------------------+-----------+-----------
 6460882231754752 | c970bdc0  | FAILED
 5771274900733952 | 6ca8506e  | FAILED
 6217757068361728 | 63ed3bc7  | FAILED
 5980650261446656 | ae283736  | FAILED
 6585898394976256 | 5f13999a  | COMPLETED
 4527474786172928 | 7f9acc9b  | COMPLETED
 4826100842364928 | e8d4e94a  | COMPLETED
 4540563027918848 | b9ee5f2d  | FAILED
 6358528648019968 | c5af141c  | FAILED
 5998005284765696 | e212a0f8  | COMPLETED
 6488580526178304 | b85d5dc0  | FAILED
 5034091344560128 | 7dc95cc3  | ABORTED
 5688692477526016 | bb048e31  | COMPLETED
 5481187977723904 | d351063e  | COMPLETED
 5101831568752640 | f30848cb  | COMPLETED <-- the change
 6395317408497664 | 3f33b63d  | COMPLETED
 6741325208354816 | 877ae5db  | COMPLETED
 4594007789010944 | de746e0d  | COMPLETED
 6497208998035456 | 461b8cc9  | COMPLETED