~EventuallyPersistentEngine() should use the objects owned by it only before deleting them

~EventuallyPersistentEngine() will invoke all the destructors (KVBucket, DCPConnMap, etc..) and these destructors will invoke other destructors.

However, most of the objects in kv-engine hold a reference to the EventuallyPersistentEngine obj. It is quite likely that the destructors of those objects use the EventuallyPersistentEngine obj. For example: ~EventuallyPersistentEngine() --> ~DCPConnMap() --> ~Producer() --> ~ActiveStream() and there we try to get the reference to the 'Vbucket' using the 'KVBucket' object in EventuallyPersistentEngine obj.

However, if ~EventuallyPersistentEngine() has already deleted its 'KVBucket' then we could crash on a nullptr access (http://review.couchbase.org/#/c/84652/4/engines/ep/src/ep_engine.cc was to avoid one such crash found in unit tests).

So, we should revisit on the order in which the EventuallyPersistentEngine objects are deleted in ~EventuallyPersistentEngine() and carefully have an explicit order that we want.