Feature: return new events in long-polled /sync calls

[olivia-fl]

Currently, if there aren't any new events at the beginning of a long-polled /sync call, we just wait for the timeout (capped to 30s) before returning an empty response. The implementation for this behavior is here. What we should be doing instead is watching for new events while we wait for the timeout, and returning a response with new events immediately if any occur. I suspect that fixing this will be a noticeable improvement in message delivery latency for most clients.