[core] limits: hard low difficulty limit, future timestamps prevention.

 - eliminate future timestamp effect ; will not prevent the massive
   miners we currently see increasing the difficulty, as they're able to
   mine multiple blocks at 5 or 10 times the real network diff...
 - ugly hardcoded difficulty lowlimit to stop returning crazy
   difficulties after being hashrate abused, killing nodes
Merge branch 'dev-0.1.3' into release-0.1.3