Add async RPC queue and operation classes.

Add z_getoperationstatus RPC command.
Add z_sendmany RPC command (dummy implementation, does not send actual
coins).

src/rpcclient.cpp

@@ -98,6 +98,8 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "zcbenchmark", 1 },
     { "zcbenchmark", 2 },
     { "getblocksubsidy", 0},
+    { "z_sendmany", 1},
+    { "z_sendmany", 2},
     { "z_importkey", 1 }
 };