Add watch-only support to Zcash RPC methods

Balance totals do not include spends linked to viewing key addresses, as
nullifiers cannot be calculated and therefore spends cannot be detected.

src/keystore.h

@@ -174,6 +174,12 @@ public:
                 setAddress.insert((*mi).first);
                 mi++;
             }
+            ViewingKeyMap::const_iterator mvi = mapViewingKeys.begin();
+            while (mvi != mapViewingKeys.end())
+            {
+                setAddress.insert((*mvi).first);
+                mvi++;
+            }
         }
     }