Aggregation API update

From riot-ios created by [manuroe](https://github.com/manuroe): vector-im/riot-ios#2456

The CS API will follow this principle:
a) we don't bundle during sync
b) when you get a bundled event then those counts are accurate as of that API call, so don't update the counts

This has been discussed at https://matrix.to/#/!fuiphBbxmgKxJjXYFn:matrix.org/$155844046336cSPsh:jki.re?via=matrix.org&via=jki.re&via=lant.uk

- [ ] This means that the implementation just needs to update the reaction summary only on live reaction events. It does not need to update it on reaction events received while paginating.
- [ ] Optionally, for HS not supporting the aggregation API yet, we can support building reaction summary from reactions received while paginating.