Refactor some pow functions

[UdjinM6]

This moves special diff adjustment rule out DGW (cause it has nothing to do with the algo itself) and refactors GetNextWorkRequired (adding missing asserts, reordering things in a more natural way). Should have no changes in actual behaviour.

[nmarley]

Agree w/moving special diff rule outside DGW. Those asserts feel dirty to me for some reason, like a "poor-man's error checking". But this is a nit, shouldn't hold up anything.

utACK

[UdjinM6]

Those asserts feel dirty to me for some reason, like a "poor-man's error checking".

The first one was introduced in bitcoin@972714c which we backported as part of btc0.15 but it changed GetNextWorkRequiredBTC only. I'm just applying the same logic ("Assert all the things!") to Dash specific code and making one more step here (i.e. checking pblock too).

[codablock]

utACK