Convert threads koan to use bordeaux-threads

I've changed the threads koan to use bordeaux-threads instead of sb-thread for portability. The BT API is quite similar to sb-thread except for the following:

 - `bordeaux-threads:make-thread` does not accept arguments to pass to the thread function. To do this you have to use a lambda that funcalls the thread function.
 - The semaphore concept doesn't exist in BT. It does, however, have condition variables, which can be used to similar effect. It would be trivial to create a semaphore class by encapsulating a condition variable, lock and count slot.

**UPDATE**: I've created a simple semaphore class for bordeaux-threads that's quite similar to the SBCL version. So if you want to make the threads koan portable but still keep semaphores you could use that: https://github.com/ralph-moeritz/bt-semaphore