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/pow.cpp

@@ -25,7 +25,8 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
         return nProofOfWorkLimit;
 
     {
-        if (params.fPowAllowMinDifficultyBlocks)
+        if (params.nPowAllowMinDifficultyBlocksFromHeight &&
+            pindexLast->nHeight >= params.nPowAllowMinDifficultyBlocksFromHeight.get())
         {
             // Special difficulty rule for testnet:
             // If the new block's timestamp is more than 6 * 2.5 minutes