Bug Fixes Pour In as Bittorrent Engine Gets Significant Tweaks

AskAlexSharov introduced early-exit pruning for DirtySpace() in #18787, and Taratorio fixed a commitment state key bug (#18858). Prune performance improved further thanks to another fix by @anacrolix (@Copilot's deadlock fix for v3.3.5). Eastorski ensured forward compatibility with the new snapshot format (v0), and Anacrolix added an alias and changed P2P discovery flags in #18785, #18819, and #18824.