Allow multiple concurrent GET_new_events_for calls on the same user to make just one HTTP hit and share the Future result
Rewrite tests that await multiple events about other users into the same Future->needs_all await pattern the others have
Rewrite final GET_new_events-using test into using await_for_event and Future->needs_all convergence instead
No need to provide the old GET_new_events/saved_events API from 03events-initial.pl any more; much neatening of code