Optimize various operations in the ArticleStorage backends

this disables the fallback from DB to FS backend, and the other way round.
Only enabled again in TicketArticleStorageSwitch
config option Ticket::StorageModule::CheckAllBackends

for setups where not all attachements are either in the database or the file system
Merge branch 'article-storage-optimization' of https://github.com/moritz/otrs