Update changelog to mention performance improvements in write batches (#1534)
Update changelog to mention performance improvements in write batches (#1534)

Reflects #1505, #1507, #1533.
firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk