Improve cross-node algorithm in `EventHubClientLogic.FetchAsync()`: remove duplicate payload fetch

[zhabis]

Related #510

The current naive implementation fetches Event instances from ALL nodes in cohort, instead it should only fetch payload from the first and fetch only `(gdid)' from the rest. Then, it should try to merge results in a set in memory and then fetch bodies of THOSE events which were NOT returned by the first at all.

This optimization is possible because events are immutable.