MXFileStore: File corruption since the introduction of MXBackgroundSyncService

File corruption are sometimes detected by MXFileStore:
````
[MXFileStore] Warning: MXFileStore has been reset due to room file corruption. Room id: XXXX
````

What is weird is, most of the time, it happens on a room that was not used at all for the last 50 app life cycles we have in logs.
The issue started to be visible with Amandine's account from this build:
```
Element version: 1.1.3
MatrixKit version: 0.13.2
MatrixSDK version: 0.17.4
Build: develop #20201210112922
```
This is the first build we shared with the new `MXBackgroundSyncService` (https://github.com/matrix-org/matrix-ios-sdk/pull/927).

This issue was noticeable because the app failed to do initial sync afterwards (#3917).