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!
Well, fascinating!

I was basing my obviously-false-in-general claim on a test I did a long
time ago, perhaps even with gcc 3.9, so strings have changed rather a lot
since then...

On Sat, Sep 27, 2014 at 12:46 PM, HowardHinnant <notifications@github.com>
wrote:

> In src/ripple/validators/impl/SourceStrings.cpp:
>
> >      {
> >          // VFALCO TODO This can't be right...?
> > -        return beast::String::empty;
> > +        return std::string{};
>
> See http://codepad.org/M66KQyC5 which demonstrates otherwise, at least
> for libc++.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/ripple/rippled/pull/587/files#r18124545>.
>