12-hour difficulty adjustment window #451

After v20 HF, difficulty adjustment algorithm looks at 144 prior blocks (~12 hours), starting from current height minus 3 blocks (~15 minutes).

Of the 144 blocks, the 12 highest and 12 lowest block times are dropped from the calculation and the remaining 120 blocks are used to get the average block time.

The average block time is then used to adjust the difficulty level to target 5 minute blocks. If a large miner/pool jumps on the network, it will take 15 blocks (~1 hours and 15 minutes) for the algo to react, with a gradual difficulty adjustment within 135 blocks (~11 hours and 15 minutes).

V19 has a difficulty adjustment window of 720 blocks (2 days and 12 hours). If we are going to bring back pool mining in v20, we need a faster adjustment.