Buried Deployments.

Replaces old miner-vote checks for three past upgrades with the fixed block heights at which they took effect.

BIPs 34, 65 and 66 switched on once enough miners signalled support, and checking this meant looking back over the previous 1,000 blocks. Since those upgrades are long settled, nodes now treat each rule as active from a recorded block height. It is a tidy-up of node software.