Check difficulty in ContextualCheckBlockHeader

[scravy]

LegacyValidationInterface::ContextualCheckBlockHeader does not check difficulty.

#939 might fix that, or a smaller patch can just patch this in.

@Ruteri inintially reported, I'm aware of it, but lets not forget about it.

[scravy]

Actually, whether the difficulty is met is correctly checked in CheckStake, as difficulty affects eligibility of a coin. This check does not rely on nBits in the header. In fact, this value is never read. That is to say, we could actually drop it form the header.