Arcyd: change default sleep to 3 seconds

Reduce the default sleeping time from 60 seconds to 3 seconds, Arcyd is
expected to be quite interactive. A minute is too long to wait for a
response.

Test Plan:
$ ./precommit.sh
Add phlmp_cyclingpool to multiprocess cycling jobs

For multiprocessing jobs that are repeated on cycle and on occasion take
much longer than usual. Provide a CyclingPool that will fork worker
processes to complete the jobs and gather the results.

Provide a convenient generator member function so the client can simply
iterate over the results without worrying about the details.

Support leaving some workers running when they occasionally overrun the
expected run time.

Add test driver with documented test concerns.

Test Plan:
$ ./precommit.sh
Arcyd: only sync review cache delta from worker

Instead of syncing the entire review state cache in a worker for each
job, only sync the delta introduced by the current job.

This means that we'll be able to 'overrun' jobs later, as jobs will only
send what's specific to them.

Test Plan:

General coverage
$ ./precommit.sh

Create and land a review with a cycling arcyd:
$ ./testbed/arcyd/test_shell.sh
    $ ./poke.sh
    $ ./accept.sh
Arcyd: use phlmp_cyclingpool to overrun repos

Allow repo jobs to 'overrun' a cycle if they take too long, this means
that repos won't get stuck behind 'noisy neighbor' repos as long as
there are enough worker processes available.

Test Plan:

General coverage
$ ./precommit.sh

Exercise overrun, install sleep in test_shell, pre-receive hook
$ ./testbed/arcyd/test_shell.sh
    $ cp ../origin/hooks/post-update ../origin/hooks/pre-receive
    $ vim ../origin/hooks/pre-receive
    .. write 'sleep 5' hook ..
    $ cd ../arcyd
    $ vim configfile
    .. set "--overrun-secs" to 5 ..
    $ arcyd stop
    $ arcyd start
    $ tail -f var/log/info var/log/git-phab-writes.log savereport.txt &
    $ cd ../dev
    $ ./poke.sh
    .. watch the job overrun ..
    $ ./accept.sh
    .. watch the job overrun ..
Refactor, remove unused phlmp_pool