pgsql: Add a new wait state and use it when sending data in the apply w

Add a new wait state and use it when sending data in the apply worker.

d9d7fe68d3 made use of an existing wait event when sending data from the
apply worker, but we should have invented a new wait event since this is a
new place to wait.

This patch corrects the mistake by using a new wait event
"LogicalApplySendData".

Author: Hou Zhijie
Reviewed-by: Peter Smith
Discussion: https://postgr.es/m/CA+TgmobWzbr9H3yN3dLVckviEZKemPwd+XyCFKEgyZQZhgP66Q@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/fce003cfde219b7016140f83f67ebcfdf75aa0dc

Modified Files
--------------
doc/src/sgml/monitoring.sgml                          | 5 +++++
src/backend/replication/logical/applyparallelworker.c | 3 ++-
src/backend/utils/activity/wait_event.c               | 3 +++
src/include/utils/wait_event.h                        | 1 +
4 files changed, 11 insertions(+), 1 deletion(-)