Refactor scalability mechanism - make it _container agnostic_

[kuba--]

Based on Q&A session, the current driver container management approach in Babelfish has some potential problems with compatibility across different OS.

We agreed to switch to docker API as a standard runtime for drivers.

[kuba--]

It's also partly related to #293

[dennwc]

Also note that one of the considered options is containerd, which is used by Docker under the hood. It's not as low-level as libcontainer and is more similar to Docker itself in this regard.

But running in Docker first makes a lot of sense.

[kuba--]

@bblfsh/maintainers
I moved this design to the doc: https://docs.google.com/document/d/1671ApmxFCOmerVKloVC6Q6j8BsJr8xZHbhuE40ZK-as/edit?usp=sharing

So far, there is nothing more (just copied the current proposal), but we can continue discussion in the doc (like @creachadair suggested).

[smola]

Please, consider how this will run in a Kubernetes pod and a docker-compose service. If we use Docker backend, we might effectively need a Docker in Docker setup. In any case, creating a runtime interface sounds great and I hope it might help with rootless containers at some point.

[creachadair]

We discussed this a bit more in https://github.com/src-d/minutes/pull/778. While there are important design elements we still need to investigate, we broadly agreed that our target state should:

1. Separate scalability support from basic operation,
2. Delegate scalability to other components (e.g., Kubernetes), but
3. Support running locally for simple workloads without requiring users to install complex dependences (e.g., minikube),
4. Not require privileged containers (per above, see also Run containers in rootless mode #153 for context), and
5. (Nice to have, if practical) Be runnable locally even without containerization.

Some of the issues we'll need to cover are in the minutes and the associated doc.

[kuba--]

I made this task as epic/umbrella task. Following sub-tasks may also be tracking tasks, so if it's needed we can split them as well, as needed:

• "Design/Prototype scalability and deployment support": Design/Prototype scalability and deployment support #319
• "Design/Prototype local scalable environment": Design/Prototype local scalable environment #320
• "Prototype native parser scaling mechanism": Prototype native parser scaling mechanism #321
• "Test scalability layers": Test scalability layers #322
• "Discovery mechanism": Drivers discovery mechanism #323