Fix attachment downloading
Fix attachment downloading

Fix issue where an attachment will only be downloaded once per
program instance not per class instance. This was caused by using
dispatch_once which should only be used for singleton set up

jimix/CDTDatastore,phatblat/CDTDatastore,jhs/CDTDatastore,jhs/CDTDatastore,phatblat/CDTDatastore,cloudant/CDTDatastore,cloudant/CDTDatastore,jhs/CDTDatastore,jimix/CDTDatastore,cloudant/CDTDatastore,jimix/CDTDatastore,phatblat/CDTDatastore,cloudant/CDTDatastore