Arcyd: refactor, remove useless on_review_event

The functionality of abdt_logging.on_review_event() was removed in a
previous commit. Clear up remaining references to it.

Test Plan:
$ ./precommit.sh
Arcyd: make io_log access thread-safe

Trivially add a threading.Lock around access to
abt_logging.on_io_event() internals, in preparation for enabling
multi-threaded arcyd.

Test Plan:
$ ./precommit.sh
Arcyd: add thread name to log messages for clarity

Make it easier to tell what's happening on which thread by putting the
name of the thread into the message.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ exit
    .. look at var/log/info output in trace ..