Introduce structured concurrency in the mapper

[vaibhavtiwari33]

@vigith The fix added as part of this PR is still valid.

can you open an issue stating what is the "right way" to solve it? I believe that the solution is to use structured concurrency.

Originally posted by @vigith in #3267 (comment)

[vigith]

why re-introduce? did we lose it during refactoring? /cc @yhl25

[vaibhavtiwari33]

Actually, no, I think I was misremembering while creating the issue that we lost structured concurrency when we introduced map tasks. From the PR it seems we always spawned short-lived tokio tasks even before the refactor.