[testnet] diff,adjustment: EMA retargetting.

 - Exponential Moving Average diff computation, over the last 2160
   blocks duration (roughly 3 days worth of blocks).
 - active on testnet since block 5182, not enabled on livenet yet.
 - testnet will eventually allow diff=1 mining, when last block was
   found more than 10 mins ago ; livenet will divide diff by two under
   same circumstances.
 - will probably be activated on livenet in a short future according to
   tests.