pgsql: Fix assertion failure in snapshot building

Fix assertion failure in snapshot building

Clear any potential stale next_phase_at value from the snapshot
builder which otherwise may trip an assertion check ensuring
that there is no next_phase_at value.

This can be reproduced by running 80 concurrent sessions like
the below where $c is a loop counter (assumes there has been
1..$c databases created) :

echo "
CREATE TABLE replication_example(id SERIAL PRIMARY KEY,
somedata int,
text varchar(120));
SELECT 'init' FROM
pg_create_logical_replication_slot('regression_slot_$c',
'test_decoding');
SELECT data FROM
pg_logical_slot_get_changes('regression_slot_$c', NULL,
NULL, 'include-xids', '0',
'skip-empty-xacts', '1');
" | psql -d regress_$c >>psql.log &

This was originally committed as 48efb23 and backpatched down to
v16, but since then there have been reports of this happening on
v14 and v15 as well so this is a backpatch of 48efb23 down to 14.

Bug: #17695
Author: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Reviewed-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Reported-by: bowenshi <zxwsbg(at)qq(dot)com>
Reported-by: Alexander Pyhalov <a(dot)pyhalov(at)postgrespro(dot)ru>
Reported-by: Teja Mupparti
Discussion: https://postgr.es/m/17695-6be9277c9295985f@postgresql.org
Backpatch-through: v14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7be51eb4e169672d2029d955cb776e2252e6b7d3

Modified Files
--------------
src/backend/replication/logical/snapbuild.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)