Use thread pool for DeleteDanglingObjectStorageFilesStep

[kmichel-aiven]

CPU intensive steps running in the main thread block all async operations.

This include other concurrent requests, leaving astacus completely unresponsive.

Start moving some steps entirely in a thread pool to avoid that.

We were already using the thread pool a lot anyway, since our object storage library is sync, but now instead of entering/leaving the pool for every small operation, we run an entire step as a single task.

[DDB-951]