QueryNodeImpl is too mutable [LUCENE-5676]

We use query nodes both during parsing and as a method of building queries. Somehow a bug has crept into our system where a QueryNode somehow gets mutated to have a parent when the node we created didn't originally have a parent. Some processor in our pipeline treats nodes differently based on the parent, which makes it a very hard to find bug.

It would be very helpful if there were a method like freeze() which would make the instance immutable from thereon, something like the following:

```Java
private volatile boolean frozen;

public final boolean isFrozen() {
return frozen;
};

protected void aboutToMakeChanges() {
if (isFrozen()) {
throw new IllegalStateException("This object is frozen");
}
}

public A freeze() {
frozen = true;
return this;
}

// ...

private void setParent(QueryNode parent) {
aboutToMakeChanges();
// existing code
}
```

---
Migrated from [LUCENE-5676](https://issues.apache.org/jira/browse/LUCENE-5676) by Trejkaz

Start by tracing QueryNodeImpl and the query-node processing pipeline, focusing on how parent relationships are assigned during parsing and query construction. Determine the scope of the proposed freeze behavior and how callers should use it; done means frozen nodes cannot be mutated and the parent-related regression is covered by appropriate tests.

Written by the indexing model from the issue text.