4.9.0 - Patience is a Virtue

Add

• Add a waitForCache option to allowing cache callback called after cache operation finishes. #963

Fix

• Animated image now will recognize .once and .finite(1) the same thing. #982
• Replace class-only protocol keyword with AnyObject as Swift convention. #983
• A wrong cache callback parameter when storing cache with background decoding. #986
• Access downloadHolder in a serial queue to avoid racing. #984