Fix WAL flush LSN used by logical walsender during shutdown

Commit 6eedb2a5fd8 made the logical walsender call
XLogFlush(GetXLogInsertRecPtr()) to ensure that all pending WAL is flushed,
fixing a publisher shutdown hang. However, if the last WAL record ends at
a page boundary, GetXLogInsertRecPtr() can return an LSN pointing past
the page header, which can cause XLogFlush() to report an error.

A similar issue previously existed in the GiST code. Commit b1f14c96720
introduced GetXLogInsertEndRecPtr(), which returns a safe WAL insertion end
location (returning the start of the page when the last record ends at a page
boundary), and updated the GiST code to use it with XLogFlush().

This commit fixes the issue by making the logical walsender use
XLogFlush(GetXLogInsertEndRecPtr()) when flushing pending WAL during shutdown.

Backpatch to all supported versions.

Reported-by: Andres Freund <[email protected]>
Author: Anthonin Bonnefoy <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Discussion: 
https://postgr.es/m/vzguaguldbcyfbyuq76qj7hx5qdr5kmh67gqkncyb2yhsygrdt@dfhcpteqifux
Backpatch-through: 14

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/8ee536c89517c0d2fc9df2177eb1e51e61367a68

Modified Files
--------------
src/backend/replication/walsender.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

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