abdt_logging: use multiprocessing.Lock()

Replace the threading.Lock for the IO log with a multiprocess one,
paving the way toward multiprocessing.

Test Plan:
$ ./precommit.sh
arcyd/acceptance_tests: touch logs before cat

In the case where we're selectively disabling some tests, some logs may
not be present. Touch them before cat-ing to ensure they exist.

Test Plan:
.. selectively disable some tests ..
$ ./testbed/arcyd/acceptance_tests.sh