Move UpdateTime to pow
Move CBlockIndex::GetBlockWork() to pow::GetProofIncrement(nBits)
replace ComputeMinWork with CheckMinWork
Merge pull request #4377

654871d replace ComputeMinWork with CheckMinWork (jtimon)
b343c1a Move CBlockIndex::GetBlockWork() to pow::GetProofIncrement(nBits) (jtimon)
c2c02f3 Move UpdateTime to pow (jtimon)