Make media service dupe check scalable

[muratcakir]

GetCachedFilesByFolderAsync preloads ALL file metadata in a folder to perform fast dupe checks. This is fast, but doesn't scale well (folders with 1M files and up!).

Implement IMediaDupeDetector interface with two implementations:

1. Preload like before
2. No preload --> read DB per query

IMediaDupeDetectorFactory decides which impl is better for the current payload. It then instantiates and returns the impl as transient dependency. Input is folderId.