Change CNode::askFor to a deque

AskFor() guarantees that entries will always be added with a monotonically increasing timestamp. 

```
    int64_t nNow = GetTimeMicros() - 1000000;
    static int64_t nLastTime;
    ++nLastTime;
    nNow = std::max(nNow, nLastTime);
    nLastTime = nNow;
```

This means that there is no need to use a map as event queue. This avoids some overhead. Also limit the maximum size of the structure.
(see #4547 for further discussion)