Fix syncing issues

Fixes syncing issues caused by global change to the difficulty retarget.
Recommended hard fork

Hard fork at block 14984 to make it match the specs of retargeting every
4 blocks. The block number is chosen to coincide with Balthazar's
retarget average kicking in at block 15000 so that there are 4 retarget
intervals of the correct spacing to scan at this point.
Add commenting for the recommended changes
Merge pull request #1 from Petr1fied/master

Fix syncing issues