Collections are ready when sync interface marks it ready

[KyleAMathews]

We've been using the first commit as when the collection is ready but that has two issues:

• for the initial sync, you have to know now to commit any transactions until all data is written — which isn't very intuitive or ergonomic as you might have to special sync write code for the first sync
• it's fairly common to need to do blank commits atm e.g. if the backend has no data or if there's an error when loading — it's odd doing a blank commit vs. changing marking the collection as errored or ready.

Perhaps isReady() for the function name.