Use blackcoin's new checkpoints.cpp/h

[denravonska]

Our CheckSync code is a huge part of the synchronization time. Compared to blackcoin we spend most of our time in there:

  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 89.47    301.71   301.71     3999     0.08     0.08  Checkpoints::CheckSync(uint256 const&, CBlockIndex const*)   <- Gridcoin
  0.00     40.97     0.00     2000     0.00     0.00  Checkpoints::CheckSync(int) <- Blackcoin

Copy the new checkpoints.cpp/h files from blackcoin, keeping our actual checkpoints of course. The major changes are:

• Remove relaying of checkpoints.
• Greatly improved CheckSync implementation.

[denravonska]

Fixed with #678.