Add Semaphore, Condition, Event

Dask currently has some distributed coordination primitives like Locks and Queues that match the API of normal local Locks and Queues. There are other primitives like these that we do not yet support, notable `Semaphore`, `Condition`, and `Event`.

These might be good training for people who want to become familiar with extending the dask.distributed system. The files `lock.py`, `queues.py`, and `variable.py` might be a good model for development here.