[Event Hubs] Revisit load-balancing algorithm initialization

[chradek]

Currently, EventProcessor will grab 1 partition at a time during load-balancing. Depending on the number of processors, it can take a long time before all partitions are assigned a processor.

Another strategy was to take all available partitions and allow other processors to steal as necessary.

[richardpark-msft]

Customer has a case where their initialization per partition is expensive so stealing and constantly cycling partitions hurts their performance. This makes the greedy approach a non-starter for their use-case.