Proposed DIGI implementation

[ny2cafuse]

3 files modified, one being the readme just because the KGW line has been bugging me.

DIGI coded to kick in at block 180000 as a place holder. Can be changed to match a desired date and time.

-Fuse

[ny2cafuse]

Checkpoints need to be added as well. I forgot to mention that.

[GeertJohan]

Thanks for these changes. It looks like GetNextWorkRequired_DIGI is never called with these patches applied. A case is missing for when DiffMode = 4. Also I think the protocol version should not be increased, as the protocol will remain compatible. I will cherry pick some changes and squash to a single commit.

[ny2cafuse]

You are correct. I added the Diffmode case statement, but forgot the call to the digi function. Thanks for catching that. I edited the code from Github instead of pulling it from my local repo. Should be complete other than that.

As far as protocol version goes, you need to be able to reject "valid" blocks from unupdated clients. If this can be done in a different way, so be it. I just know this is an easy way to reject blocks submitted by unupdated wallets using the old algo.

-Fuse

[ny2cafuse]

Added missing function call.

-Fuse