Coinselection prunes extraneous inputs from ApproximateBestSubset

This is a combination of 3 commits.

- Coinselection prunes extraneous inputs from ApproximateBestSubset
  A further pass over the available inputs has been added to ApproximateBestSubset after a candidate set has been found. It will prune any extraneous inputs in the selected subset, in order to decrease the number of input and the resulting change.
- Moved set reduction to the end of ApproximateBestSubset to reduce performance impact
- Added a test for the pruning of extraneous inputs after ApproximateBestSet

Github-Pull: #4906
Rebased-From: 5c03483e26 af9510e037 fc0f52d780

src/wallet/wallet.cpp

@@ -1632,6 +1632,16 @@ static void ApproximateBestSubset(vector<pair<CAmount, pair<const CWalletTx*,uns
             }
         }
     }
+
+    //Reduces the approximate best subset by removing any inputs that are smaller than the surplus of nTotal beyond nTargetValue. 
+    for (unsigned int i = 0; i < vValue.size(); i++)
+    {                        
+        if (vfBest[i] && (nBest - vValue[i].first) >= nTargetValue )
+        {
+            vfBest[i] = false;
+            nBest -= vValue[i].first;
+        }
+    }
 }
 
 bool CWallet::SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int nConfTheirs, vector<COutput> vCoins,