1.2.1 with ASIC invalid share fix

Adds workarounds for invalid (low diff) shares on IceRiver / Bitmain ASICs, so they can continue selling their broken-ass devices without putting in the effort to fix the problem themselves.

Default settings (var_diff: true, shares_per_min: 20) should now be fine for all devices. For current ASICs, you'll need to set the existing parameter 'extranonce_size: 2' as before, as well as a new parameter 'pow2_clamp: true'. See the README and/or config.yaml for further details.