Added fPowNoRetargeting field to Consensus::Params that disables nBits recalculation.

src/consensus/params.h

@@ -22,6 +22,7 @@ struct Params {
     /** Proof of work parameters */
     uint256 powLimit;
     bool fPowAllowMinDifficultyBlocks;
+    bool fPowNoRetargeting;
     int64_t nPowTargetSpacing;
     int64_t nPowTargetTimespan;
     int64_t DifficultyAdjustmentInterval() const { return nPowTargetTimespan / nPowTargetSpacing; }