[core] target,adjustment: enabled ema-based target calculation.

 - retargetting now occurs at every block
 - new target calculated using exponential moving average block duration
   over last 2160 blocks (roughly last 3 days).
 - should allow quick exit from current stalled state by extra-reducing
   difficulty on longer blocks.
 - difficulty increments/cap reduced.
 - MANDATORY on block 101631