Allow mining min diff for very slow (2h+) blocks (#2175)

Author: UdjinM6

src/pow.cpp

@@ -109,6 +109,11 @@ unsigned int static DarkGravityWave(const CBlockIndex* pindexLast, const CBlockH
         if (pindexLast->nChainWork >= UintToArith256(uint256S("0x000000000000000000000000000000000000000000000000003ff00000000000"))
             // and immediately on devnet
             || !params.hashDevnetGenesisBlock.IsNull()) {
+            // recent block is more than 2 hours old
+            if (pblock->GetBlockTime() > pindexLast->GetBlockTime() + 2 * 60 * 60) {
+                return bnPowLimit.GetCompact();
+            }
+            // recent block is more than 10 minutes old
             if (pblock->GetBlockTime() > pindexLast->GetBlockTime() + params.nPowTargetSpacing*4) {
                 arith_uint256 bnNew = arith_uint256().SetCompact(pindexLast->nBits) * 10;
                 if (bnNew > bnPowLimit) {