EventBuilder now clears itself between `build_sync_response` calls so
that each subsequent call will return an empty response if nothing was
added.
This allows reuse of a single EventBuilder instance which is important
for correct sync token rotation.
This allows us to hold onto an EventBuilder object and use it to build
multiple sync responses. Previously this would have resulted in each
of the responses having the same next_batch sync token. This would make
clients ignore the latter responses if they have already received any of
the previous ones.
