Be more robust when opening realm DBs

We should reset DB when it fails to open to avoid the app crashes in loop.

Proposed solution before I lost the code:
```
- (nullable RLMRealm*)realm
{
NSError *error;

RLMRealmConfiguration *realmConfiguration = self.realmConfiguration;
RLMRealm *realm = [RLMRealm realmWithConfiguration:self.realmConfiguration error:&error];

if (error)
{
NSLog(@"[MXRealmAggregationsStore] realmForUser gets error: %@", error);

NSLog(@"[MXRealmAggregationsStore] realmForUser: Retrying...");
[NSFileManager.defaultManager removeItemAtURL:realmConfiguration.fileURL error:nil];
realmConfiguration = self.realmConfiguration;
realm = [RLMRealm realmWithConfiguration:self.realmConfiguration error:&error];
if (realm)
{
NSLog(@"[MXRealmAggregationsStore] realmForUser: Realm db fixed");
}
else
{
NSLog(@"[MXRealmAggregationsStore] realmForUser: Still failing: %@", error);
}
}

return realm;
}
```

Start by locating the realm method that uses RLMRealmConfiguration and RLMRealm realmWithConfiguration:error:. Reproduce or trace the failed database-opening path, then verify that a failed open is handled by removing the database and retrying, with a successful retry returning a realm instead of leaving the app in a crash loop.

Written by the indexing model from the issue text.