Identify undefined behaviors around ContinueAsNew and provide semantics for them

**Is your feature request related to a problem? Please describe.**
There exist some `ContinueAsNew` scenarios where the semantics are unclear. Some of them are already described here:
https://github.com/Azure/durabletask/pull/480

These make it difficult to diagnose and debug user problems, which is ultimately the problem we want to avoid.

**Describe the solution you'd like**
It would be good to identify a list of these _tricky_ / undefined cases and to decide on a semantics for them. We could then validate that our implementation is doing the right thing and fix it if not.

We can discover these scenarios either by hand, or using automated tools like fuzzers or model-checkers.

**Describe alternatives you've considered**
N/A

**Additional context**
This issue was recently brought up here: https://github.com/Azure/durabletask/pull/480