[core] difficulty,fix: limit effects of future-ntime blocks.

 - thanks to the reporter, we first had something like this on first
   implementation, but it did not make it to the live build :(

 - note that network is really weak right now and abuse attempts come
   with enough power to disrupt the chain :(
Merge branch 'dev-0.1.3' into release-0.1.3
[core] logging: log remote ip address while receiving a block.

 - can't do much, if you are able to mine every single block at 5 to 10
   times the usual net diff ... you won :)
Merge branch 'dev-0.1.3' into release-0.1.3
Merge branch 'release-0.1.3'
checkpoint added
Merge branch 'dev-0.1.3' into release-0.1.3
Merge branch 'release-0.1.3'
revert.
Merge branch 'dev-0.1.3' into release-0.1.3
Merge branch 'release-0.1.3'
[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
Merge branch 'release-0.1.3'
Merge branch 'master' of https://github.com/terracoin/terracoin into terracoin