[C++] Add ConcatMap, MergeMap, and an async-reentrant version of Map #27725
Description
These are all needed for the current ARROW-7001 work. I'm breaking them out on their own so they can be reviewed in isolation.
Reporter: Weston Pace / @westonpace
Assignee: Weston Pace / @westonpace
PRs and other links:
Note: This issue was originally created as ARROW-11883. Please see the migration documentation for further details.