rpc: clean up the RPCFunc constructor signature (#7586)

Instead of taking a comma-separated string of parameter names, take each
parameter name as a separate argument. Now that we no longer have an extra flag
for caching, this fits nicely into a variadic trailer.

* Update all usage of NewRPCFunc and NewWSRPCFunc.

rpc/jsonrpc/test/main.go

@@ -13,7 +13,7 @@ import (
 )
 
 var routes = map[string]*rpcserver.RPCFunc{
-	"hello_world": rpcserver.NewRPCFunc(HelloWorld, "name,num"),
+	"hello_world": rpcserver.NewRPCFunc(HelloWorld, "name", "num"),
 }
 
 func HelloWorld(ctx context.Context, name string, num int) (Result, error) {