pgsql: Teach walsender to update its process title for replication comm

Teach walsender to update its process title for replication commands.

Because the code path taken for SQL commands executed in a walsender
will update the process title, we pretty much have to update the
title for replication commands as well. Otherwise, the title shows
"idle" for the rest of a logical walsender's lifetime once it's
executed any SQL command.

Playing with this, I confirm that a walsender now typically spends
most of its life reporting
walsender postgres [local] START_REPLICATION
Considering this in isolation, it might be better to have it say
walsender postgres [local] sending replication data
However, consistency with the other cases seems to be a stronger
argument.

In passing, remove duplicative pgstat_report_activity call.

Discussion: https://postgr.es/m/880181.1600026471@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/babef40c9a999949abe0ae8e82240cac3f154237

Modified Files
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src/backend/replication/walsender.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)