* Clarify the "asset not found" log during thumbnail generation: it's about database
* Move not found sidecars to verbose level instead of "old=null, new=null" at debug
* Log memory creation at default level
* Add explicit log for missing exif date time
Instead of: Date and time is undefined using exifTag undefined for asset ...
* Log database migration start/end at default level
Currently, these messages are logged as "debug". But they are not printed
when debug or verbose level is set. This is due to the known limitation:
SystemConfigService sets LogLevel later on, after migrations run.