Create commands after payload conversion

[cretz]

What was changed

Before this change, the Python SDK added commands on the activation completion before payload conversion and then filled them out including with conversion. This meant that in cases where the payload converter caused a application error (or was caught), the command would be half created/filled-in and you'd get errors like "invalid TaskQueue on ScheduleActivityTaskCommand: missing task queue name".

Now, in multiple places, we do the payload conversion before command creation/addition which allows exceptions to occur before the command is added.

Checklist

1. Closes #540
2. Closes #564