We are changing how the Delta API works, as discussed:
1. We have removed the `updated` and `removed` arrays and now keep
everything in the `events` array.
2. We decided to keep the hydration cache separate from the events array
internally. Since the hydration cache has a special structure and may
contain not just one feature but potentially 1,000 features, it behaved
differently, requiring a lot of special logic to handle it.
3. Implemented `nameprefix` filtering, which we were missing before.
Things still to implement:
1. Segment hydration and updates to it.
Our delta API was returning archived feature as updated. Now making sure
we do not put `archived-feature `event into `updated` event array.
Also stop returning removed as complex object.
