Remove the use of beast::String from rippled (RIPD-443)

Friday night insomnia special: almost completely excise `beast::String` from rippled. Small remnants remain, most related to `Config` which will go away when that refactor goes through, and a couple related to JSON.

This will be a fun review @rec, @tdfischer!
STAmount::getText() already returns a std::string. I don't think you need to wrap the existing ""s in std::string.