aio: Infrastructure for io_method=worker

This commit contains the basic, system-wide, infrastructure for
io_method=worker. It does not yet actually execute IO, this commit just
provides the infrastructure for running IO workers, kept separate for easier
review.

The number of IO workers can be adjusted with a PGC_SIGHUP GUC. Eventually
we'd like to make the number of workers dynamically scale up/down based on the
current "IO load".

To allow the number of IO workers to be increased without a restart, we need
to reserve PGPROC entries for the workers unconditionally. This has been
judged to be worth the cost. If it turns out to be problematic, we can
introduce a PGC_POSTMASTER GUC to control the maximum number.

As io workers might be needed during shutdown, e.g. for AIO during the
shutdown checkpoint, a new PMState phase is added. IO workers are shut down
after the shutdown checkpoint has been performed and walsender/archiver have
shut down, but before the checkpointer itself shuts down. See also
87a6690cc6.

Updates PGSTAT_FILE_FORMAT_ID due to the addition of a new BackendType.

Reviewed-by: Noah Misch <noah@leadboat.com>
Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Co-authored-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt
Discussion: https://postgr.es/m/20210223100344.llw5an2aklengrmn@alap3.anarazel.de
Discussion: https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m

src/backend/postmaster/launch_backend.c

@@ -48,6 +48,7 @@
 #include "replication/slotsync.h"
 #include "replication/walreceiver.h"
 #include "storage/dsm.h"
+#include "storage/io_worker.h"
 #include "storage/pg_shmem.h"
 #include "tcop/backend_startup.h"
 #include "utils/memutils.h"
@@ -197,6 +198,7 @@ static child_process_kind child_process_kinds[] = {
 	[B_ARCHIVER] = {"archiver", PgArchiverMain, true},
 	[B_BG_WRITER] = {"bgwriter", BackgroundWriterMain, true},
 	[B_CHECKPOINTER] = {"checkpointer", CheckpointerMain, true},
+	[B_IO_WORKER] = {"io_worker", IoWorkerMain, true},
 	[B_STARTUP] = {"startup", StartupProcessMain, true},
 	[B_WAL_RECEIVER] = {"wal_receiver", WalReceiverMain, true},
 	[B_WAL_SUMMARIZER] = {"wal_summarizer", WalSummarizerMain, true},