Initial syncs disconnect too quickly

Due to some trouble with my homeserver I had to clear cache and login again. However, I've not been able to complete an initial sync for several days. I've noticed that the logs for synapse show that Android will close a HTTP request after 30s, which may be one of the reasons why i'm having so much trouble. I'd expect the connection to stay open for around the same time as Element web, which is ~80 seconds.

The requests are cached between /sync requests, so the work isn't wasted per se but it would be good if the timeout was longer to avoid additional traffic.

I'm still trying to determine why my initial sync never completes for Android.

Synapse log:
```
2021-02-28 15:27:00,721 - synapse.rest.client.v2_alpha.sync - 184 - INFO - GET-150994- Client has disconnected; not serializing response.
2021-02-28 15:27:00,721 - synapse.http.server - 650 - WARNING - GET-150994- Not sending response to request , already disconnected.
2021-02-28 15:27:00,722 - synapse.access.http.8018 - 299 - INFO - GET-150994- 86.16.94.189 - 8018 - {@Half-Shot:half-shot.uk} Processed request: 35.616sec/-10.126sec (22.589sec, 2.020sec) (28.486sec/105.193sec/4789) 0B 200! "GET /_matrix/client/r0/sync?filter=181&set_presence=offline&timeout=0 HTTP/1.0" "Element/1.1.0 (Linux; U; Android 11; Pixel 3a Build/RQ1A.210105.00; Flavour GooglePlay; MatrixAndroidSDK_X 0.0.1)" [30 dbevts]
```