feat: use multiple processes to publish layers

[astuyve]

What does this PR do?

1. Revives some old code which spins off child processes (with &) and tracks the pids up to a batch size to make backfilling layers faster
2. removes the backfill confirmation logic and instead warns:
3. Removes the region arg, we already ask/confirm so if they don't pass it, we don't need to choose it:

Motivation

I've never said "no" to a backfill, and I can't think of a condition where we would say no unless we inadvertently used a large number for the target version (eg 999 instead of 99). This script could simply be killed and started again with the lower number, as lambda will only let us increment versions upwards anyways.

Testing Guidelines

Additional Notes

Types of Changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (fbe67f7) 81.16% compared to head (f2b3d6c) 81.16%.

:exclamation: Current head f2b3d6c differs from pull request most recent head 6d97464. Consider uploading reports for the commit 6d97464 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #438 +/- ## ======================================= Coverage 81.16% 81.16% ======================================= Files 50 50 Lines 2039 2039 Branches 466 466 ======================================= Hits 1655 1655 Misses 323 323 Partials 61 61 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.