cosmetic fix - renaming signal/slot to lock CoverDelegate - using "onlyCachedCoverArt" instead

it is just a small fix to make it more readable... the logic keeps the same, but the name makes more sense...
CoverArtCache - fixing bug introduced by the last changes (renaming to "onlyCached")
CoverArtCache - holding md5hash from the queue if it contains the trackId
- it improves the performance (a lot) because it return the cover faster (as it's already loaded, it does not need to wait the db operation)