Send batches over channels

[EVODelavega]

The core often sends events to the broker as a batch (e.g. settlements or trades). Optional subscribers (like the ones used for the event stream) rely on an internal channel to receive these events one by one. If we send a batch of trade events to the subscriber, this means the broker will iterate over each event trying to send each trade event one by one. This may seem harmless at first, but if there's a lot of streams open, especially if some of them are behind (ie the internal channel buffer is full), this leads to a lot of overhead, increase CPU usage, and actual data/events making it out to the subscribers slower and slower. With this change, batched events from the core make their way to the subscribers as a batch, and do so quickly (as fast as a single event). How the subscribers process the individual events from that point on is less of a concern to the core/broker, and doesn't impact performance as much.