Asynchronous Backing

[eskimor]

Asynchronous backing allows collators to build parachain blocks off of relay chain parents other than the most recent block. This will quadruple parachain block production time and halve parachain latency, greatly increasing total network throughput and enabling future features for more flexible blockspace offerings like on-demand parachains and elastic scaling.

Spec & tracking issue
Project board

Q2 milestones:

• Merge node-side code to master in compatibility mode (assuming audit returns with no severe issues)
• Finish collator-side code (Aura alterations for Asynchronous Backing compatibility cumulus#2476)
• Test on Versi (Test compatibility mode on Versi polkadot#7064)
• Measure total network throughput before and after async backing (Async backing integration & load test suite polkadot-sdk#706)

Q3 milestones:

• Test on Rococo
• Deploy to Kusama
• Deploy to Polkadot