I am facing a problem with stress testing SOLR-5944, even though I think this problem persists in Solr even without my changes.

The symptom is that during a stress test (similar to TestStressReorder), RTG gets a document which is older version than that of the last acknowledged write.

Possible reason:
```(DUH2's commit())
...
1:      if (cmd.softCommit) {
2:        // ulog.preSoftCommit();
3:        synchronized (solrCoreState.getUpdateLock()) {
4:          if (ulog != null) ulog.preSoftCommit(cmd);
5:          core.getSearcher(true, false, waitSearcher, true);
6:          if (ulog != null) ulog.postSoftCommit(cmd);
7:        }
8:        callPostSoftCommitCallbacks();
9:      }
...
```

* Before line 1, there was an update (say id=2) which was in ulog's map. Maps are, say, `map={2=LogPtr(1234)}, prevMap={...}, prevMap2={...}`
* Due to line 4 (ulog.preSoftCommit()), the maps were rotated. Now, the id=2 is in prevMap: `map={}, prevMap={2=LogPtr(1234)}, prevMap2={...}`. Till now RTG for id=2 will work.
* Due to line 5, a new searcher is due to be opened. But this is asynchronous, and lets assume this doesn't complete before few more lines are executed.
* Due to line 6 (ulog.postSoftCommit()), the previous maps are cleared out. Now the maps are: `map={}, prevMap=null, prevMap2=null`
* If there's an RTG for id=2, it will not work from the ulog's maps, so it will fall through to be searched using the last searcher. But, the searcher due to be opened in line 5 hasn't yet been opened. In this case, the returned document will be whatever version of id=2 that was present in the previous searcher.

I cannot seem to access JIRA at the moment, so cannot file a bug right now. Can someone please confirm if this is a valid problem? If so, any suggestions for a fix, please? (I tried opening a ulog.openRealtimeSearcher() in the above synchronized block, but the problem still persists).