Incorrect initialization order in WiredTigerKVEngine

In the WiredTigerKVEngine, an incorrect initialization order of private fields can lead to a race condition at shutdown. The KVEngine's SizeStorer is depended on by the JournalFlusher and so should be initialized beforehand to get the proper destruction order on shutdown.

In addition, shutdown calls for components of the KVEngine in WiredTigerKVEngine::cleanShutdown() should be reordered to match the actual order of destruction.