v0.21.2-rc1

• Make --sticky-endgames on by default (still off in training) (#844)
• update download links in README (#842)
• Recalibrate centipawn formula (#841)
• Also make parents Terminal if any move is a win or all moves are loss or draw. (#822)
• Use parent Q as a default score instead of 0 for unvisited pv. (#828)
• Add stop command to selfplay interactive mode to allow for graceful exit. (#810)
• Increased hard limit on batch size in opencl backend to 32 (#807)