pgsql: pg_stat_activity: show NULL stmt start time for walsenders

pg_stat_activity: show NULL stmt start time for walsenders

Returning a non-NULL time is pointless, sinc a walsender is not a
process that would be running normal transactions anyway, but the code
was unintentionally exposing the process start time intermittently,
which was not only bogus but it also confused monitoring systems looking
for idle transactions.  Fix by avoiding all updates in walsenders.

Backpatch to 11, where walsenders started appearing in pg_stat_activity.

Reported-by: Tomas Vondra
Discussion: https://postgr.es/m/20191209234409.exe7osmyalwkt5j4@development

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/896db774e5b0a851792f02c8a19ea9b07c9dcc23

Modified Files
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src/backend/access/transam/xact.c | 7 +++++++
1 file changed, 7 insertions(+)