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 
Ah, you're probably right about the RVO!

I'll fix that...