[Event Hubs] Ensure names of receivers are unique

[ramya-rao-a]

In Event Hubs library, users can currently provide a name for the receiver that gets created when they initiate a receive call. This name is internally used in a cache that we maintain for receivers which is used to recover broken receivers

Since users can provide the same name when using receive against different partitions, our internal cache gets to a bad state resulting in not being able to recover from broken receivers.

[chradek]

Closed by #6506