Fix RLock's entries

 - Changed the semaphore initial value to 0, since every time
    we try to acquire it is because we subscribed and know we have
    to wait for another thread to signal it. This way, we just acquire
    it once, and avoid useless operations.
 - Since the value was 1, the first thread to subscribe would go through
    the subscribe() method twice, meaning the entry's counter would be raised
    to 2, and on unsubscribe() decremented to 1, therefore, it would
    never be removed from ENTRIES.