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.m...@gmail.com>
Reviewed-by: Alexander Lakhin <exclus...@gmail.com>
Reported-by: bowenshi <zxw...@qq.com>
Reported-by: Alexander Pyhalov <a.pyha...@postgrespro.ru>
Reported-by: Teja Mupparti
Discussion: https://postgr.es/m/17695-6be9277c92959...@postgresql.org
Backpatch-through: v14

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0f404c5812471d21c1d006b4e5f354ca216cea22

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

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