abdi_processrepoarglist: use phlmp_pool

Replace _WorkerManager with phlmp_pool.generate_results().

This means that far fewer processes will be required to do the
processing, as generate_results() uses queues to supply work to the
workers.

Test Plan:
General coverage
$ ./precommit.sh

Exercise concurrency
$ ./proto/arcyd-tester --repo-count 100