arrow

ARROW-15929: [R] io_thread_count is actually the CPU thread count

As noted in ARROW-15929, we have code referring to the wrong thread pool (i.e., `arrow::io_thread_count()` and `arrow::set_io_thread_count()` are actually setting the number of cores used instead of the actual thread count). This appears to have been an inadvertent addition changed in ARROW-12760 (#10316).

``` r
# remotes::install_github("apache/arrow/r")
library(arrow, warn.conflicts = FALSE)

cpu_count()
#> [1] 8
set_io_thread_count(4)
cpu_count()
#> [1] 4
```

This PR switches that so that we're referring to the correct thread pool:

``` r
# remotes::install_github("paleolimbot/arrow/r@r-thread-io-pool-count")
library(arrow, warn.conflicts = FALSE)

cpu_count()
#> [1] 8
set_io_thread_count(4)
cpu_count()
#> [1] 8
io_thread_count()
#> [1] 4
```

Closes #12633 from paleolimbot/r-thread-io-pool-count

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>