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 
Just curious, wouldn't the explicit move prevent the compiler from doing RVO? Also I believe local variables are guaranteed to be moved implicitly by the standard.