optimizing pg_upgrade's once-in-each-database steps

A number of pg_upgrade steps require connecting to each database and
running a query.  When there are many databases, these steps are
particularly time-consuming, especially since this is done sequentially in
a single process.  At a quick glance, I see the following such steps:

    * create_logical_replication_slots
    * check_for_data_types_usage
    * check_for_isn_and_int8_passing_mismatch
    * check_for_user_defined_postfix_ops
    * check_for_incompatible_polymorphics
    * check_for_tables_with_oids
    * check_for_user_defined_encoding_conversions
    * check_old_cluster_subscription_state
    * get_loadable_libraries
    * get_db_rel_and_slot_infos
    * old_9_6_invalidate_hash_indexes
    * report_extension_updates

I set out to parallelize these kinds of steps via multiple threads or
processes, but I ended up realizing that we could likely achieve much of
the same gain with libpq's asynchronous APIs.  Specifically, both
establishing the connections and running the queries can be done without
blocking, so we can just loop over a handful of slots and advance a simple
state machine for each.  The attached is a proof-of-concept grade patch for
doing this for get_db_rel_and_slot_infos(), which yielded the following
results on my laptop for "pg_upgrade --link --sync-method=syncfs --jobs 8"
for a cluster with 10K empty databases.

    total pg_upgrade_time:
    * HEAD:  14m 8s
    * patch: 10m 58s

    get_db_rel_and_slot_infos() on old cluster:
    * HEAD:  2m 45s
    * patch: 36s

    get_db_rel_and_slot_infos() on new cluster:
    * HEAD:  1m 46s
    * patch: 29s

I am posting this early to get thoughts on the general approach.  If we
proceeded with this strategy, I'd probably create some generic tooling that
each relevant step would provide a set of callback functions.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com