facts: proof-of-concept server-side statistics.

Publishes statistics from the server to a Minimongo collection,
Facts.server. (On the server side, Facts is *not* backed by MongoDB.) By
default, this is available to all users if autopublish is on, or no
users if it is not on, but you can configure this with
Facts.setUserIdFilter.

You can add an ugly view on the stats to your app with {{> serverFacts }}.

Current stats, by package:

- mongo-livedata:
  - observe-handles
  - live-results-sets (when this is less than observe-handles,
    cursor de-dup is helping you)
- livedata
  - subscriptions
  - crossbar-listeners
  - sessions