pgsql: Enable IO concurrency on all systems

Enable IO concurrency on all systems

Previously effective_io_concurrency and maintenance_io_concurrency could not
be set above 0 on machines without fadvise support. AIO enables IO concurrency
without such support, via io_method=worker.

Currently only subsystems using the read stream API will take advantage of
this. Other users of maintenance_io_concurrency (like recovery prefetching)
which leverage OS advice directly will not benefit from this change. In those
cases, maintenance_io_concurrency will have no effect on I/O behavior.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://postgr.es/m/CAAKRu_atGgZePo=_g6T3cNtfMf0QxpvoUh5OUqa_cnPdhLd=gw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2a5e709e721cf5f890cde51755b84cfe25d1c4d9

Modified Files
--------------
doc/src/sgml/config.sgml | 16 +++++++-------
doc/src/sgml/ref/alter_tablespace.sgml | 2 +-
doc/src/sgml/ref/create_tablespace.sgml | 2 +-
src/backend/access/common/reloptions.c | 8 -------
src/backend/commands/variable.c | 30 ---------------------------
src/backend/utils/misc/guc_tables.c | 4 ++--
src/backend/utils/misc/postgresql.conf.sample | 4 ++--
src/bin/initdb/initdb.c | 5 -----
src/include/storage/bufmgr.h | 6 ------
src/include/utils/guc_hooks.h | 4 ----
10 files changed, 14 insertions(+), 67 deletions(-)