Add phlmp_pool for concurrent job processing

Add a new generator function for yielding the results from a list of
callables, which are to be called concurrently in different processes.

Add a breathing test.

Test Plan:
$ ./precommit.sh
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
abdt_differ: fix docstr, raises LargeDiffException

Fix the docstring for abdt_differ.make_raw_diff, the section on what's
raised was incorrect and incomplete.

Test Plan:
General coverage
./precommit.sh