clear __currentUserId when puppet stops

[hcfw007]

In this issue, https://github.com/wechaty/puppet/issues/184, we discussed that we should not call puppet.logout() in puppet.stop(). That works well, however recently when studying logs about puppet resetting, we found that after reseting the puppet is still logged in before the new login event comes, and that might be problematic. Thus beside emiting a logout event, we should also clear __currentUserId in puppet.stop(). I'll create a PR to show that.