Tracking Issue: Do not assume Substrate Runtime execution model

[pepyakin]

Right now, sc-executor crate assumes a specific execution model.

Specifically:

• that there is a sequence of Substrate Runtimes
• This sequence is appended each time there is a new runtime.
• Only a handful of runtimes are used at a moment of time
• We don't fear non determinism, so we don't clear the runtime instances.

We reuse sc-executor for Polkadot's Parachain Validation Function (PVF) execution and that actually requires a bit different model. It has to deal with a lot more instances at the same time. It's sensitive to non determinism and cannot reuse the instances.

See for more details: https://hackmd.io/c9GhdTjpT-y0W-w8v8CknA

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.