This adds to the list of concurrency models we may want to implement (along side a WorkerPool, GeventWorker, AsyncWorker and MultiprocessWorker), also related to #45. Some work being done on #1803.

We replicated all tests that Worker went through using the ThreadpoolWorker, that means the basic Worker test suite, dependencies tests, callbacks tests and registry tests. Most of the tests pass, an overview on the issues so far (I haven't spend time looking into them):

Failed tests:

• Timeout & Failed registry: a job that times out doesn't seem to be sent to the FailedJobRegistry
• Dependency's Race Condition: we don't seem to be able to get the job.result when job's being executed
• Main dependency test fails: jobs seems to be stuck at queued while we should be finished
• Dependencies are met at execution time: we can't seem to get the job result

Other todo's:

• Test the worker is not eager: doesn't take more jobs than the available idle threads (this should be working fine, but should be better tested)
• Save the worker class and the pool size on the worker registry
• Add the scheduler (not implemented)
• Any other thread-safety issues we want to test?

Any help on this is very much appreciated!