This makes the serialization of events synchronous (and it no
longer access the database), but we must manually calculate and
provide the bundled aggregations.
Overall this should cause no change in behavior, but is prep work
for other improvements.
Due to updates to MSC2675 this includes a few fixes:
* Include bundled aggregations for /sync.
* Do not include bundled aggregations for /initialSync and /events.
* Do not bundle aggregations for state events.
* Clarifies comments and variable names.
MSC3030: https://github.com/matrix-org/matrix-doc/pull/3030
Client API endpoint. This will also go and fetch from the federation API endpoint if unable to find an event locally or we found an extremity with possibly a closer event we don't know about.
```
GET /_matrix/client/unstable/org.matrix.msc3030/rooms/<roomID>/timestamp_to_event?ts=<timestamp>&dir=<direction>
{
"event_id": ...
"origin_server_ts": ...
}
```
Federation API endpoint:
```
GET /_matrix/federation/unstable/org.matrix.msc3030/timestamp_to_event/<roomID>?ts=<timestamp>&dir=<direction>
{
"event_id": ...
"origin_server_ts": ...
}
```
Co-authored-by: Erik Johnston <erik@matrix.org>
