pgsql: Introduce a maintenance_io_concurrency setting.

Introduce a maintenance_io_concurrency setting.

Introduce a GUC and a tablespace option to control I/O prefetching, much
like effective_io_concurrency, but for work that is done on behalf of
many client sessions.

Use the new setting in heapam.c instead of the hard-coded formula
effective_io_concurrency + 10 introduced by commit 558a9165e08. Go with
a default value of 10 for now, because it's a round number pretty close
to the value used for that existing case.

Discussion: https://postgr.es/m/CA%2BhUKGJUw08dPs_3EUcdO6M90GnjofPYrWp4YSLaBkgYwS-AqA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fc34b0d9de27ab5ac2887da00b3892bdabd89e45

Modified Files
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doc/src/sgml/config.sgml | 20 ++++++++++++++++++++
doc/src/sgml/ref/alter_tablespace.sgml | 11 +++++++----
doc/src/sgml/ref/create_tablespace.sgml | 11 +++++++----
src/backend/access/common/reloptions.c | 16 +++++++++++++++-
src/backend/access/heap/heapam.c | 16 +++-------------
src/backend/storage/buffer/bufmgr.c | 7 +++++++
src/backend/utils/cache/spccache.c | 14 ++++++++++++++
src/backend/utils/misc/guc.c | 32 ++++++++++++++++++++++++++++++++
src/bin/psql/tab-complete.c | 2 +-
src/include/commands/tablespace.h | 1 +
src/include/storage/bufmgr.h | 1 +
src/include/utils/spccache.h | 1 +
12 files changed, 109 insertions(+), 23 deletions(-)