Non-deterministic workflow code can send incorrect WFT completion without task failure

[dandavison]

#1835 repros a situation in which non-deterministic workflow code sends an incorrect WFT completion without task failure, causing a invalid update state transition in History service.

The test causes a workflow to execute two WFTs. On WFT 2, it changes a boolean flag in the workflow making the code non-deterministic.

WFT1 is triggered by an update-with-start (update ID A) and is responded to by accepting & completing that update.

WFT2 is triggered by an update (update ID B).

However, when responding to WFT2, the SDK re-sends acceptance and completion messages for update A, which is incorrect.