While working on some async rewrites in ema I realized that changes one sends from EBook/CalBackend aren't propagated to UI after e_data_cal/book_view_notify_complete is called, which makes the asynchronous fetching very hard to achieve.

Imagine situation:
 - EDataBook/CalView begun, return to it everything from cache, and indicate
   completeness of it

 - after some time, while view is still active, one receives an update from
   a server, and calls e_book_backend_notify_update/
   e_book_backend_notify_remove, thus expecting all data views being notified
   about new change, if appropriate for it, but no, bad luck, these changes
   are cached in the factory unless there's "enough" of them.

I would suppose to have a batched update based on time, rather than on a count, so nothing is missed. Or the combination, but definitely with some upper time limit, to not have missing items in the UI.

Say not keeping the cache for more than a second or three?

(My initial use-case, how I discovered this, was to populate view by cache and tell it it's complete, and then invoke new thread and fetch changes from a server, notifying about new contacts by e_book_backend_notify_update as expected, but as my contact list had only 8 items then I didn't see them, until next start of evolution, or calling notify_complete again, which is more than strange. I changed it to not "notify_complete" so early, which also allows to see progress messages in a status bar, but the above scenario about later updates still can be valid.)