when doing a limited sync, and a new fragment is created,
this._lastLiveKey is updated immediately. If the transaction
would then fail, the fragmentId in this._lastLiveKey was incremented
but the fragment wasn't written to the store, so if sync is resumed
and would subsequently succeed, fragmentIds would be assigned to events
that don't have a corresponding fragment in the timelineFragment store.
This would throw errors when trying to load the timeline,
breaking the whole app.
This changes SyncWriter to only update this._lastLiveKey in
the emit phase, when the transactions has been committed already.
This is a suboptimal implementation now,
but is the API we want to end up with. Readers and Writers
in persistence add fragments to the comparer when they become
aware of, create, or link up fragments.
