Fixes to query unsubscribe issues.
Finish fixes to query unsubscribe. Introduce Model#alwaysSubscribe

Model#alwaysSubscribe will keep the passed in subscriptions around. The
intention here is to isolate some subscribes that should always be in scope
(e.g., a logged in user)
Require noop via racer-util/fn
Fixes to txn application and loading data from a fetch or subscribe

- Don't apply txn speculatively or post txn ack if document does not exist.
- Loading data from a fetch or subscribe should invalidate the spec model, so
  the mutation (applied via model._memory) is surfaced to the spec model via a
  spec model re-calculation
Ensure cache invalidation before another update directly to Model#_memory._data
log some context when throwing inside model.fn
Merge branch 'master' of git://github.com/codeparty/racer

Conflicts:
	lib/descriptor/util.js