pgsql: Fix race condition in test_decoding "slot" test.

Fix race condition in test_decoding "slot" test.

This test, just added in commit a924c327e, sometimes fails because
the old backend hasn't finished dropping the temporary replication slot
when the new backend looks. Borrow the previously-invented methodology
for waiting for the old process to disappear from pg_stat_activity.

Petr Jelinek

Discussion: https://postgr.es/m/62935e6f-4f1b-c433-e0fa-7f936a38b3e5@2ndquadrant.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/23f722ba8e19ca1a7c2ada9d6e687989b6e8f4d1

Modified Files
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contrib/test_decoding/expected/slot.out | 19 ++++++++++++++-----
contrib/test_decoding/sql/slot.sql | 16 ++++++++++++----
2 files changed, 26 insertions(+), 9 deletions(-)