Proposal: Out-of-Order NOTIFY via GUC to Improve LISTEN/NOTIFY Throughput

failed: regress/async, regress/without_overlaps, regress/rules, regress/psql, regress/psql_crosstab, regress/psql_pipeline, regress/amutils, regress/stats_ext, regress/collate.linux.utf8, regress/collate.windows.win1252, regress/select_parallel, regress/write_parallel, regress/vacuum_parallel, regress/maintain_every, regress/publication, regress/subscription, regress/select_views, regress/portals_p2, regress/foreign_key, regress/cluster, regress/dependency, regress/guc, regress/bitmapops, regress/combocid, regress/tsearch, regress/tsdicts, regress/foreign_data, regress/window, regress/xmlmap, regress/functional_deps, regress/advisory_lock, regress/indirect_toast, regress/equivclass, regress/json, regress/jsonb, regress/json_encoding, regress/jsonpath, regress/jsonpath_encoding, regress/jsonb_jsonpath, regress/sqljson, regress/sqljson_queryfuncs, regress/sqljson_jsontable, regress/plancache, regress/limit, regress/plpgsql, regress/copy2, regress/temp, regress/domain, regress/rangefuncs, regress/prepare, regress/conversion, regress/truncate, regress/alter_table, regress/sequence, regress/polymorphism, regress/rowtypes, regress/returning, regress/largeobject, regress/with, regress/xml, regress/partition_join, regress/partition_prune, regress/reloptions, regress/hash_part, regress/indexing, regress/partition_aggregate, regress/partition_info, regress/tuplesort, regress/explain, regress/compression, regress/compression_lz4, regress/memoize, regress/stats, regress/predicate, regress/numa, regress/oidjoins, regress/event_trigger, regress/event_trigger_login, regress/fast_default, regress/tablespace, pg_waldump/001_basic, pg_upgrade/002_pg_upgrade, dblink/dblink, tcn/tcn