Storing absolute path for different configs breaks compatibility on iOS
as app's dir is changed after upgrade. The solution is to store relative
paths and to concatenate it with `backend.rootDataDir`. The only
exception is `LogFile` as it is stored outside `backend.rootDataDir` on
Android. `LogDir` config was added to allow adding of custom dir for log
file.
Configs concerned:
`DataDir`
`LogDir`
`LogFile`
`KeystoreDir`
`BackupDisabledDataDir`
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