Implement batch loading of event sourcing aggregates (Flows, Tasks)

[zaychenko-sergei]

Currently we do not support batch loading of flows & tasks, and load them in sequential loops like below:

            // TODO: implement batch loading
            for flow_id in relevant_flow_ids {
                let flow = Flow::load(flow_id, self.flow_event_store.as_ref()).await.int_err()?;
                yield flow.into();
            }

Knowing a set of queries (id's in our case), it should be possible to load all related events in 1 SQL query in chronological order. Then event_sourcing crate could post-process the loaded collection of events, and re-construct multiple aggregates in 1 call.

This should optimize database roundtrips, esp. for listing flows:

[s373r]

Cool, tracing is the best visual way to know that optimization is needed.

Yes, we knew this before, but when you can see this need on the graph, superb!

[zaychenko-sergei]

@demusdev Consult with @rmn-boiko for detailed supervision