Fixes NSInternalInconsistencyException on multiple sections update

If you have a table/collection view with multiple sections and the data for more than one section changes at once, it causes an NSInternalInconsistencyException. This is because CommonReactiveSource calls PerformBatchUpdates on every updated section once, but the collection/tableview expects all the updates to be batched in one call to PerformBatchUpdates. 
I have a manual test case where the crash can be reproduced and this patch fixes it (I can attach the manual test case here, but couldn't figure out how to do it).

Is there any way we can combine section and item updates? Every time we call `Buffer` we're kicking off timers, so we want to minimize how many we kick off