Arcyd: add rudimentary multithreading

Start to move towards a multithreading model by delegating all real
work to a worker thread shortly after 'arcyd start'.

Immediately try to join the worker thread.

Note some of the problems that will need to be addressed before spawning
more workers.

Test Plan:

Try to get general coverage
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

Exercise shutdown paths and inspect logs
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd
    $ arcyd stop
    $ tail var/log/*