Add network stats to metrics screen

Closes #1688

src/rpcmining.cpp

@@ -37,7 +37,7 @@ using namespace std;
  * or over the difficulty averaging window if 'lookup' is nonpositive.
  * If 'height' is nonnegative, compute the estimate at the time when a given block was found.
  */
-Value GetNetworkHashPS(int lookup, int height) {
+int64_t GetNetworkHashPS(int lookup, int height) {
     CBlockIndex *pb = chainActive.Tip();
 
     if (height >= 0 && height < chainActive.Height())