Revisit the Substrate deployment

[rakyll]

The new deployment model requires:

• AX server become a MT front door
• We introduce a new binary to run the harness or the environment as an isolated actor
• We remove axepp and make Substrate calls directly from the AX server
• Allow bringing a custom container as an environment per harness on Substrate

[rakyll]

Partially blocked by #2.

[rakyll]

I'm going to work on some scaffolding changes for the new deployment model, and we can split the work from there:

1. MT AX server (the new frontdoor as a ReplicaSet)
2. Placeholder HarnessService
3. axharness binary that implements the HarnessService
4. Controller2 + new logic to create/resume and suspend by conversation_id
5. Potential changes to the event log