Fix ServiceConnection DDP negotiation

Now ServiceConnection's guarantee is that once a DDP connection is
successfully negotiated, it won't restart. This relies on the assumption
that the only use of reconnect({_force: true}) is DDP protocol
negotiation!

Drop some unnecessary (and flawed, for this application) `disconnect`
stream events.

Also, remove some unnecessary `new` calls.

Fixes 'meteor mongo some-galaxy app'.