Presence filter in sync ignored

This issue has been migrated from [#2298](https://github.com/matrix-org/synapse/issues/2298).

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Hi, my initial sync with matrix.org is typically > 2 MB and I found most of the data is in presence events. According to client-server spec, I should be able to specify a Filter to limit the number of presence events https://matrix.org/docs/spec/client_server/r0.2.0.html#get-matrix-client-r0-sync

```json
{
"presence": {
"limit": 10
}}
```
Unfortunately, this has no effect on the returned data. I also tried to use a room timeline filter and that works fine.

Being able to reduce the sync to a smaller size would be useful for mobile clients and also help reduce the load on matrix.org