Introduce tri-state concurrency locks

[jankotek]

JDBM currently uses ReadWriteLock and plain synchronized. This is not enough for our needs. We should use tri-state locks:

• read
• read-for-write
• write

It should behave similar as ReadWriteLock, but read-for-write should not block other read, but should block other read-for-write. Example usage:

public V ConcurrentMap.putIfAbsent(K key, V value) {
    lock.readWriteLock().lock();
    try{
        V oldKey = get(key)
        if(oldKey!=null)
           return oldKey;

        lock.writeLock.lock();
        try{
            return put(key, value);
        }finally{
            lock.writeLock.unlock()
        }    
    }finally {
        lock.readWriteLock().unlock();
    }
}

[jankotek]

Closing, not going to fix right now