Including endpoints in saves for offsets

[knordstrom]

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

I marked feature but neither really fit. Technically this is a third category, ie.

• [x] Breaking change that provides enhanced functionality

Description of the change

Offsets are now saved with a key that includes the IoTHub endpoint so that multiple IoTHubs may be tracked using the same datastore.

Motivation for the change

See discussion here https://github.com/Azure/toketi-iothubreact/issues/24

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This change is 

[knordstrom]

Sorry about the long delay, there were other things that took me away for a bit. It looks to me like this was ready but the hashing we agreed to in the discussion was not implemented. Is that accurate or was there something else?