Update difficulty on every block

My patch will cause difficulty to be updated for every algorithm on every block, not just the algorithm that was solved.  In particular, the difficulty of one algorithm may decrease when a different algorithm is solved.  This should help with the wild fluctuations currently seen in most of the algorithms.

This will require a hard fork, which I tentatively set at block 215000, about a week from now.

I don't think Myriad needs this.  Their difficulty adjustment algorithm is much less aggressive than DigiShield and they don't have the oscillations.