Cancellations

[slinkydeveloper]

This issue includes:

• Introduce the distinction fallible journal entries/actionable journal entries #50
• Introduce cancelled/failed field for background invoke and resolve awakeable
• Introduce failed completion variant for GetState
• Introduce StartMessage.cancel field
• Describe the SDK cancellation behavior in the protocol spec

[tillrohrmann]

I guess we can exclude #50 and points 2. and 3. from the scope of this issue.

[slinkydeveloper]

I think it still makes sense to retain the #50, as it clarifies the behavior of those entries wrt runtime retries.