Null checks for MultiMap methods

I used NullPointerException because IMap already uses that exception when a user passes null value as parameter. However I'm not sure whether it should be IllegalArgumentException or not.
Extract this to a method, so you get:

```
 public void unlock(K key) {
assertKeyNotNull(key);
final Data keyData = toData(key);
MultiMapUnlockRequest request = new MultiMapUnlockRequest(keyData, ThreadUtil.getThreadId(), name);
invoke(request, keyData);
}
```

Else you get a lot of syntactical noise.