sprint() recently became more strict, throwing on sprint("%s", 5). Replace
with the more modern format().
Mechanically converted with https://github.com/avikivity/unsprint.
After open-source-parsers/jsoncpp@42a161f commit jsoncpp's version
of valueToQuotedString no longer fits our needs, because too many
UTF-8 characters are unnecessarily escaped. To remedy that,
this commit provides our own string quoting implementation.
Reported-by: Nadav Har'El <nyh@scylladb.com>
Refs #3622