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/rpcclient.cpp

@@ -103,9 +103,12 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "zcbenchmark", 1 },
     { "zcbenchmark", 2 },
     { "getblocksubsidy", 0},
+    { "z_listaddresses", 0},
     { "z_listreceivedbyaddress", 1},
     { "z_getbalance", 1},
     { "z_gettotalbalance", 0},
+    { "z_gettotalbalance", 1},
+    { "z_gettotalbalance", 2},
     { "z_sendmany", 1},
     { "z_sendmany", 2},
     { "z_sendmany", 3},