pgsql: Ensure BackgroundWorker struct contents are well-defined.

Ensure BackgroundWorker struct contents are well-defined.

Coverity complained because bgw.bgw_extra wasn't being filled in by
ApplyLauncherRegister(). The most future-proof fix is to memset the
whole BackgroundWorker struct to zeroes. While at it, let's apply the
same coding rule to other places that set up BackgroundWorker structs;
four out of five had the same or related issues.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b6dd1271281ce856ab774fc0b491a92878e3b501

Modified Files
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src/backend/access/transam/parallel.c | 2 +-
src/backend/replication/logical/launcher.c | 6 ++++++
src/test/modules/test_shm_mq/setup.c | 1 +
src/test/modules/worker_spi/worker_spi.c | 2 ++
4 files changed, 10 insertions(+), 1 deletion(-)