Removed redundant code for processing PING request

The code for processing the case of a PING request is redundant of the code following the switch statement. request.cxid = -2 in the case of a PING. 

The difference between `break`ing and `return`ing is that the timing and effective execution order of the following calls is changed to:

```
zks.getZKDatabase().getDataTreeLastProcessedZxid();
zks.serverStats().updateLatency(request.createTime);
cnxn.updateStatsForResponse(request.cxid, lastZxid, lastOp,
                    request.createTime, System.currentTimeMillis());
```

From the current effective execution order:

```
zks.serverStats().updateLatency(request.createTime);
cnxn.updateStatsForResponse(request.cxid, request.zxid, lastOp,
                    request.createTime, System.currentTimeMillis());
zks.getZKDatabase().getDataTreeLastProcessedZxid();
```