Only enable min-difficulty blocks on testnet from a particular height

The min-difficulty change is a bilateral consensus rule change, and so
must be conditionally enabled in order for the earlier section of the
chain to synchronise.

Technically this could be implemented as a network upgrade, but as this will
never be deployed to mainnet, a targeted fork will suffice.

src/consensus/params.h

@@ -8,6 +8,8 @@
 
 #include "uint256.h"
 
+#include <boost/optional.hpp>
+
 namespace Consensus {
 
 /**
@@ -91,7 +93,7 @@ struct Params {
     NetworkUpgrade vUpgrades[MAX_NETWORK_UPGRADES];
     /** Proof of work parameters */
     uint256 powLimit;
-    bool fPowAllowMinDifficultyBlocks;
+    boost::optional<uint32_t> nPowAllowMinDifficultyBlocksFromHeight;
     int64_t nPowAveragingWindow;
     int64_t nPowMaxAdjustDown;
     int64_t nPowMaxAdjustUp;