Add thread parameter to solveequihash benchmark

Closes #1147

src/rpcclient.cpp

@@ -96,6 +96,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "zcrawjoinsplit", 3 },
     { "zcrawjoinsplit", 4 },
     { "zcbenchmark", 1 },
+    { "zcbenchmark", 2 },
     { "getblocksubsidy", 0}
 };