Validate the single writer enforcement

[rakyll]

AX server was able to enforce single writer easily when running in a Substrate actor, as a built-in capability. As the server is becoming multi-tenant, we need to revisit how we can enforce single writer tenancy if we want to provide it out of the box.

[rakyll]

As a pragmatic choice, we can turn this into a client responsibility. We initially assumed that it'd be cool to demonstrate Substrate through this enforcement but if we think it's much better to leave synchronization to the client, it's a viable alternative.