Revert "Get rid of consensus.fPowAllowMinDifficultyBlocks."

This reverts commit dffc025d38.

src/consensus/params.h

@@ -91,6 +91,7 @@ struct Params {
     NetworkUpgrade vUpgrades[MAX_NETWORK_UPGRADES];
     /** Proof of work parameters */
     uint256 powLimit;
+    bool fPowAllowMinDifficultyBlocks;
     int64_t nPowAveragingWindow;
     int64_t nPowMaxAdjustDown;
     int64_t nPowMaxAdjustUp;