arrow

ARROW-5504 [R]: move use_threads argument to global option

At this point the function is not exported or documented and threads are always used, users would need to set `options(arrow.use_threads)` to turn them off.

Author: Romain Francois <romain@rstudio.com>

Closes #4515 from romainfrancois/ARROW-5504/use_threads and squashes the following commits:

445364ab <Romain Francois> s/as_tibble()/as.data.frame()/
4afa7d4d <Romain Francois> + option_use_threads() function