Ingestion: Replace multiprocessing.Pool with concurrent.futures.ProcessPoolExecutor

[tasansal]

This PR replaces multiprocessing.Pool with concurrent.futures.ProcessPoolExecutor for improved exception handling and code simplification. The main issue addressed is the hanging of the ingestion process due to unhandled exceptions in subprocesses.

The changes include:

1. Removal of trace_worker_map and header_scan_worker_map wrapper functions in _workers.py, as they are no longer necessary.
2. Refactoring of blocked_io.py to use ProcessPoolExecutor.map instead of Pool.starmap. The parallel_inputs iterator has been moved directly into the map call.
3. Updating parsers.py to use ProcessPoolExecutor instead of Pool. The executor.map method is now used, with the header_scan_worker function and the input iterators passed directly as arguments.