Add a check to command modules to ensure that they're only started once. (#329)

Add a check to command modules to ensure that they're only started once. (#329)

* Add a check to command modules to ensure that they're only started once.

Wasm command modules should only be called once per instance, because
the programming model doesn't leave linear memory in a reusable state
when the program exits. As use cases arise for loading wasm modules in
environments that want to treat them like reactors, add a safety check
to ensure that command modules are used according to their
expectations.