pgsql: pg_upgrade: Parallelize retrieving loadable libraries.

pg_upgrade: Parallelize retrieving loadable libraries.

This commit makes use of the new task framework in pg_upgrade to
parallelize retrieving the names of all libraries referenced by
non-built-in C functions. This step 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/46cad8b31927410a99894279ca9af4e0d5c185b6

Modified Files
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src/bin/pg_upgrade/function.c | 71 +++++++++++++++++++++++++++----------------
1 file changed, 45 insertions(+), 26 deletions(-)