pgsql: Improve stability of TAP test for synchronous replication

Improve stability of TAP test for synchronous replication

Slow buildfarm machines have run into issues with this TAP test caused
by a race condition related to the startup of a set of standbys, where
it is possible to finish with an unexpected order in the WAL sender
array of the primary.

This closes the race condition by making sure that any standby started
is registered into the WAL sender array of the primary before starting
the next one based on lookups of pg_stat_replication.

Backpatch down to 9.6 where the test has been introduced.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera, Noah Misch
Discussion: https://postgr.es/m/20190617055145.GB18917@paquier.xyz
Backpatch-through: 9.6

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7d81bdc8c0ce838efa248928065e9b2da829f981

Modified Files
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src/test/recovery/t/007_sync_rep.pl | 42 +++++++++++++++++++++++++++++--------
1 file changed, 33 insertions(+), 9 deletions(-)