pgsql: Introduce framework for parallelizing various pg_upgrade tasks.

Introduce framework for parallelizing various pg_upgrade tasks.

A number of pg_upgrade steps require connecting to every database
in the cluster and running the same query in each one. When there
are many databases, these steps are particularly time-consuming,
especially since they are performed sequentially, i.e., we connect
to a database, run the query, and process the results before moving
on to the next database.

This commit introduces a new framework that makes it easy to
parallelize most of these once-in-each-database tasks by processing
multiple databases concurrently. This framework manages a set of
slots that follow a simple state machine, and it uses libpq's
asynchronous APIs to establish the connections and run the queries.
The --jobs option is used to determine the number of slots to use.
To use this new task framework, callers simply need to provide the
query and a callback function to process its results, and the
framework takes care of the rest. A more complete description is
provided at the top of the new task.c file.

None of the eligible once-in-each-database tasks are converted to
use this new framework in this commit. That will be done via
several follow-up commits.

Reviewed-by: Jeff Davis, Robert Haas, Daniel Gustafsson, Ilya Gladyshev, Corey Huinker
Discussion: https://postgr.es/m/20240516211638.GA1688936%40nathanxps13

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/40e2e5e92b7da358fb45802b53c735d25a51d23a

Modified Files
--------------
doc/src/sgml/ref/pgupgrade.sgml | 6 +-
src/bin/pg_upgrade/Makefile | 1 +
src/bin/pg_upgrade/meson.build | 1 +
src/bin/pg_upgrade/pg_upgrade.h | 21 ++
src/bin/pg_upgrade/task.c | 443 +++++++++++++++++++++++++++++++++++++++
src/tools/pgindent/typedefs.list | 5 +
6 files changed, 474 insertions(+), 3 deletions(-)