Improve RPCUtils by removing string copies and DecodeBase64.

The first commit is a clean - it just removes unused code and restricts columns to 80.

The second one gets rid of redundant string copies.

The third commit replaces our own supposedly-suboptimal base64 conversion with a seemingly-better one.

Reviewers: @HowardHinnant @nbougalis 
I'm with @mekishizufu - I'd avoid the `std::move` since without it the compiler will either do copy-elision or use `std::move` if that's avaiable, before considering doing a copy construction.