Prevent rescans of folders by updating the parent's storage_mtime

updating the paren't storage_mtime should prevent the folder to be rescanned after a file is written to the folder.

(partial?) fix for #5548, #3118 and possible other etag/scanning related issues.

cc @DeepDiver1975 @karlitschek 

@japaol can you check if this fixes #3118 for you?