arrow

ARROW-18012: [R] Make map_batches .lazy = TRUE by default (#14521)

This makes the default `map_batches()` behaviour lazy (i.e., the function is called once per batch as each batch arrives):

``` r
library(arrow, warn.conflicts = FALSE)
#> Some features are not enabled in this build of Arrow. Run `arrow_info()` for more information.

source <- RecordBatchReader$create(
  record_batch(a = 1:10),
  record_batch(a = 11:20)
)

mapped <- map_batches(source, function(x) {
  message("Hi! I'm being evaluated!")
  x
}, .schema = source$schema)

as_arrow_table(mapped)
#> Hi! I'm being evaluated!
#> Hi! I'm being evaluated!
#> Table
#> 20 rows x 1 columns
#> $a <int32>
```

<sup>Created on 2022-10-26 with [reprex v2.0.2](https://reprex.tidyverse.org)</sup>

This was previously a confusing default since piping the resulting `RecordBatchReader` into an `ExecPlan` would fail for some ExecPlans before ARROW-17178 (#13706). This PR commits to the (more optimal/expected) lazy behaviour.

Authored-by: Dewey Dunnington <dewey@voltrondata.com>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>