This is implementing the segments events for delta API. Previous version
of delta API, we were just sending all of the segments. Now we will have
`segment-updated` and `segment-removed `events coming to SDK.
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.