server sync dead loop

```

2018-01-20 23:01:46.690126+0800 Riot[83626:2424896] [MXHTTPClient] Background task #436 started
2018-01-20 23:01:46.691465+0800 Riot[83626:2426176] [MXFileStore commit] lasted 2ms for 1 rooms state
2018-01-20 23:01:46.720817+0800 Riot[83626:2426176] [MXFileStore commit] lasted 2ms for metadata
2018-01-20 23:01:46.721959+0800 Riot[83626:2424896] [MXFileStore commit] Background task #435 is complete - lasted 33ms
2018-01-20 23:01:46.835469+0800 Riot[83626:2424896] [MXHTTPClient] cleanupBackgroundTask
2018-01-20 23:01:46.835727+0800 Riot[83626:2424896] [MXHTTPClient] Background task #436 is complete
2018-01-20 23:01:46.835984+0800 Riot[83626:2424896] [MXSession] Received 1 joined rooms, 0 invited rooms, 0 left rooms in 147ms
2018-01-20 23:01:46.836241+0800 Riot[83626:2424896] [MXFileStore commit] Background task #437 started
2018-01-20 23:01:46.836322+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveRoomsState for 1 rooms
2018-01-20 23:01:46.836436+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveMetaData
2018-01-20 23:01:46.836576+0800 Riot[83626:2424896] [MXSession] Do a server sync
2018-01-20 23:01:46.837498+0800 Riot[83626:2424896] [MXHTTPClient] Background task #438 started
2018-01-20 23:01:46.838744+0800 Riot[83626:2426255] [MXFileStore commit] lasted 2ms for 1 rooms state
2018-01-20 23:01:46.840450+0800 Riot[83626:2426255] [MXFileStore commit] lasted 2ms for metadata
2018-01-20 23:01:46.841572+0800 Riot[83626:2424896] [MXFileStore commit] Background task #437 is complete - lasted 5ms
2018-01-20 23:01:46.951423+0800 Riot[83626:2424896] [MXHTTPClient] cleanupBackgroundTask
2018-01-20 23:01:46.951773+0800 Riot[83626:2424896] [MXHTTPClient] Background task #438 is complete
2018-01-20 23:01:46.952107+0800 Riot[83626:2424896] [MXSession] Received 1 joined rooms, 0 invited rooms, 0 left rooms in 116ms
2018-01-20 23:01:46.952427+0800 Riot[83626:2424896] [MXFileStore commit] Background task #439 started
2018-01-20 23:01:46.952589+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveRoomsState for 1 rooms
2018-01-20 23:01:46.952714+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveMetaData
2018-01-20 23:01:46.952942+0800 Riot[83626:2424896] [MXSession] Do a server sync
2018-01-20 23:01:46.954013+0800 Riot[83626:2424896] [MXHTTPClient] Background task #440 started
2018-01-20 23:01:46.955633+0800 Riot[83626:2426255] [MXFileStore commit] lasted 3ms for 1 rooms state
2018-01-20 23:01:46.957773+0800 Riot[83626:2426255] [MXFileStore commit] lasted 2ms for metadata
2018-01-20 23:01:46.959033+0800 Riot[83626:2424896] [MXFileStore commit] Background task #439 is complete - lasted 7ms
2018-01-20 23:01:46.983532+0800 Riot[83626:2424896] [MXHTTPClient] cleanupBackgroundTask
2018-01-20 23:01:46.983854+0800 Riot[83626:2424896] [MXHTTPClient] Background task #440 is complete
2018-01-20 23:01:46.984206+0800 Riot[83626:2424896] [MXSession] Received 1 joined rooms, 0 invited rooms, 0 left rooms in 31ms
2018-01-20 23:01:46.984480+0800 Riot[83626:2424896] [MXFileStore commit] Background task #441 started
2018-01-20 23:01:46.984619+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveRoomsState for 1 rooms
2018-01-20 23:01:46.984806+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveMetaData
2018-01-20 23:01:46.984922+0800 Riot[83626:2424896] [MXSession] Do a server sync
2018-01-20 23:01:46.985746+0800 Riot[83626:2424896] [MXHTTPClient] Background task #442 started
2018-01-20 23:01:46.986894+0800 Riot[83626:2426176] [MXFileStore commit] lasted 2ms for 1 rooms state
2018-01-20 23:01:46.988552+0800 Riot[83626:2426176] [MXFileStore commit] lasted 2ms for metadata
2018-01-20 23:01:46.989759+0800 Riot[83626:2424896] [MXFileStore commit] Background task #441 is complete - lasted 5ms
2018-01-20 23:01:47.031857+0800 Riot[83626:2424896] [MXHTTPClient] cleanupBackgroundTask
2018-01-20 23:01:47.032152+0800 Riot[83626:2424896] [MXHTTPClient] Background task #442 is complete
2018-01-20 23:01:47.032509+0800 Riot[83626:2424896] [MXSession] Received 1 joined rooms, 0 invited rooms, 0 left rooms in 48ms
2018-01-20 23:01:47.032828+0800 Riot[83626:2424896] [MXFileStore commit] Background task #443 started
2018-01-20 23:01:47.033101+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveRoomsState for 1 rooms
2018-01-20 23:01:47.033291+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveMetaData
2018-01-20 23:01:47.033448+0800 Riot[83626:2424896] [MXSession] Do a server sync
2018-01-20 23:01:47.034513+0800 Riot[83626:2424896] [MXHTTPClient] Background task #444 started
2018-01-20 23:01:47.036147+0800 Riot[83626:2426255] [MXFileStore commit] lasted 3ms for 1 rooms state
2018-01-20 23:01:47.038132+0800 Riot[83626:2426255] [MXFileStore commit] lasted 2ms for metadata
2018-01-20 23:01:47.039326+0800 Riot[83626:2424896] [MXFileStore commit] Background task #443 is complete - lasted 7ms
2018-01-20 23:01:47.181625+0800 Riot[83626:2424896] [MXHTTPClient] cleanupBackgroundTask
2018-01-20 23:01:47.182052+0800 Riot[83626:2424896] [MXHTTPClient] Background task #444 is complete
2018-01-20 23:01:47.182490+0800 Riot[83626:2424896] [MXSession] Received 1 joined rooms, 0 invited rooms, 0 left rooms in 149ms
2018-01-20 23:01:47.182950+0800 Riot[83626:2424896] [MXFileStore commit] Background task #445 started
2018-01-20 23:01:47.183160+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveRoomsState for 1 rooms
2018-01-20 23:01:47.183355+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveMetaData
2018-01-20 23:01:47.183558+0800 Riot[83626:2424896] [MXSession] Do a server sync
2018-01-20 23:01:47.184754+0800 Riot[83626:2424896] [MXHTTPClient] Background task #446 started
2018-01-20 23:01:47.186779+0800 Riot[83626:2426177] [MXFileStore commit] lasted 3ms for 1 rooms state
2018-01-20 23:01:47.189326+0800 Riot[83626:2426177] [MXFileStore commit] lasted 2ms for metadata
2018-01-20 23:01:47.190694+0800 Riot[83626:2424896] [MXFileStore commit] Background task #445 is complete - lasted 8ms
2018-01-20 23:01:47.268652+0800 Riot[83626:2424896] [MXHTTPClient] cleanupBackgroundTask
2018-01-20 23:01:47.268965+0800 Riot[83626:2424896] [MXHTTPClient] Background task #446 is complete
2018-01-20 23:01:47.269244+0800 Riot[83626:2424896] [MXSession] Received 1 joined rooms, 0 invited rooms, 0 left rooms in 86ms
2018-01-20 23:01:47.269482+0800 Riot[83626:2424896] [MXFileStore commit] Background task #447 started
2018-01-20 23:01:47.269653+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveRoomsState for 1 rooms
2018-01-20 23:01:47.269800+0800 Riot[83626:2424896] [MXFileStore commit] queuing saveMetaData
2018-01-20 23:01:47.269907+0800 Riot[83626:2424896] [MXSession] Do a server sync
2018-01-20 23:01:47.270895+0800 Riot[83626:2424896] [MXHTTPClient] Background task #448 started
2018-01-20 23:01:47.272262+0800 Riot[83626:2426173] [MXFileStore commit] lasted 2ms for 1 rooms state
2018-01-20 23:01:47.273944+0800 Riot[83626:2426173] [MXFileStore commit] lasted 2ms for metadata
2018-01-20 23:01:47.275100+0800 Riot[83626:2424896] [MXFileStore commit] Background task #447 is complete - lasted 6ms
```