Fix DarkGravityWave: Use only same block type

[bitbandi]

I created a copy from DarkGravityWave function, and add the fix to in:
It use the GetLastBlockIndex() when jump to pprev block.

The fix fork is set to 999999 block, you can change it to any value.

[CryptoCoderz]

This pull request does correct the PoW difficulty retarget.

This being said a more streamlined approach is desired, shortly after merger the fix will cleaned up with a followup commit.

Thank you bitbandi for your assistance, it is greatly valued!

[CryptoCoderz]

adding in fix for the "lowGravity" issue shortly