Fixes https://github.com/agent-substrate/substrate/issues/477 Implements
the following actor template update flow:
```
UpdateActor(): set .template = templ-v2
ResumeActor()
resumes using templ-v2
wait for readyz
if fail: return failure
.template = templ-v2; .status.current_template = templ-v1
(the next resume will attempt templ-v2 again)
set .status = STATUS_RUNNING
set .status.current_template = templ-v2
return success
.template = templ-v2; .status.current_template = templ-v2
```