demonstrate the race between r_c_vnode_worker_pool and poolboy's fsm

When a riak_core_vnode_worker finishes work, it sends checkin messages to both poolboy and riak_core_vnode_worker_pool. The latter
maintains a queue of work to be handled when there's room in the pool. As soon as RCVWP gets the checkin message, it asks poolboy if there
is a worker available (expecting that the worker just checked in will now be available).

The problem is that poolboy may receive RCVWP's message before receiving the worker's checkin message. If this happens, it will tell RCVWP
that the pool is full. RCVWP then sticks in the 'queueing' state until it receives another checkin message from a worker. Since another
checkin may never arrive, the pool may become frozen.

The test defined by worker_pool_pulse.erl demonstrates this race. Under PULSE execution, the test will fail with deadlock.

In order to run the test, you will need the pulse_otp beams from https://github.com/Quviq/pulse_otp on your path. You will also need to
compile poolboy with PULSE annotations - the bwf-pool-race branch of basho/poolboy provides this.