conversation.end() followed by conversation.begin() changes cid

Currently ConversationImpl.end() sets the cid to null. If you then begin() another conversation in the same request, you'll get a new cid, but AbstractConversationImpl will not flush the conversation to the session under than new cid, so if you propagated that new cid to the next request and try to load the conversation, it will not find it.

ConversationImpl.end() should probably save away the cid that it had, and re-use it if begin() is called again.