[ECIP-1029] Include uncles in total difficulty calculation (GHOST)

[sorpaas]

See ECIP-1029 for more information.

Note that while the GHOST protocol is published a long time ago and has been advertised to be used by Ethereum, due to the sort-of design flaw, it is actually never used by any well-known cryptocurrencies. Thus what I suggest is that we test this PR on a testnet first, make sure it runs stable, before taking measures to merge ECIP-1029 and incorporate this soft-fork into other clients.

The actual change is small while can impact some critical aspect of the consensus.

TODO:

• Keep 63 using the old total difficulty meaning.

[sorpaas]

Just some update on this. Back in November I received some comments from Yonatan Sompolinsky about our GHOST implementation and this PR. Both the original Ethereum one and #334 have some misunderstanding of how GHOST actually works. I haven't been able to follow up on this due to being busy with SputnikVM.

The next step would be to re-think a good minimal change. The thing is, we'll need to add the difficulty to uncles' successors, rather than their ancestors. That also means that we've probably also got something wrong with uncle rewards if using GHOST's protocol definition, but I'm not sure about that part yet.