pgsql: aio: Add io_method=io_uring

aio: Add io_method=io_uring

Performing AIO using io_uring can be considerably faster than
io_method=worker, particularly when lots of small IOs are issued, as
a) the context-switch overhead for worker based AIO becomes more significant
b) the number of IO workers can become limiting

io_uring, however, is linux specific and requires an additional compile-time
dependency (liburing).

This implementation is fairly simple and there are substantial optimization
opportunities.

The description of the existing AIO_IO_COMPLETION wait event is updated to
make the difference between it and the new AIO_IO_URING_EXECUTION clearer.

Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt
Discussion: https://postgr.es/m/20210223100344.llw5an2aklengrmn@alap3.anarazel.de
Discussion: https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c325a7633fcb33dbd73f46ddbbe91e95ddf3b227

Modified Files
--------------
.cirrus.tasks.yml | 3 +
doc/src/sgml/config.sgml | 8 +
src/backend/storage/aio/Makefile | 1 +
src/backend/storage/aio/aio.c | 41 ++
src/backend/storage/aio/aio_io.c | 22 ++
src/backend/storage/aio/meson.build | 1 +
src/backend/storage/aio/method_io_uring.c | 484 ++++++++++++++++++++++++
src/backend/storage/lmgr/lwlock.c | 1 +
src/backend/utils/activity/wait_event_names.txt | 4 +-
src/backend/utils/misc/postgresql.conf.sample | 3 +-
src/include/storage/aio.h | 8 +
src/include/storage/aio_internal.h | 13 +
src/include/storage/lwlock.h | 1 +
src/tools/pgindent/typedefs.list | 1 +
14 files changed, 589 insertions(+), 2 deletions(-)