Unsafe synchronization in CachingWrapperFilterHelper [LUCENE-2520]

CachingWrapperFilterHelper has unsaft synchronization as follow:

public DocIdSet getDocIdSet(IndexReader reader) throws IOException {
if (cache == null) {
cache = new WeakHashMap();
}

synchronized (cache) {
...
}

It is not safe to assgine a new object to cache before synchronizing on. it may results sycnchronize on two different object if the method is called concurrently when cache =null.

---
Migrated from [LUCENE-2520](https://issues.apache.org/jira/browse/LUCENE-2520) by Wendy Feng

Search the Java source for CachingWrapperFilterHelper and start at getDocIdSet(IndexReader reader). Examine the cache initialization and synchronization path under concurrent calls; done means the method cannot synchronize on different cache objects during first use, with the relevant existing tests passing.

Written by the indexing model from the issue text.