pgsql: pg_upgrade: Parallelize subscription check.

pg_upgrade: Parallelize subscription check.

This commit makes use of the new task framework in pg_upgrade to
parallelize the part of check_old_cluster_subscription_state() that
verifies each of the subscribed tables is in the 'i' (initialize)
or 'r' (ready) state. This check will now process multiple
databases concurrently when pg_upgrade's --jobs option is provided
a value greater than 1.

Reviewed-by: Daniel Gustafsson, Ilya Gladyshev
Discussion: https://postgr.es/m/20240516211638.GA1688936%40nathanxps13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7baa36de58bdaeb4797d592aeb9ef1db08320853

Modified Files
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src/bin/pg_upgrade/check.c | 206 ++++++++++++++++++++++++---------------------
1 file changed, 111 insertions(+), 95 deletions(-)