[C++] Add ConcatMap, MergeMap, and an async-reentrant version of Map

[asfimport]

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:

• GitHub Pull Request #9643

_{Note: This issue was originally created as ARROW-11883. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 9643
#9643