Said commands print errors as they validate the sstables. Currently this
intermingles with the regular JSON output of these commands, resulting
in ugly and confusing output.
This is not a problem for scripted use, as logs go to stderr while the
JSON go to stdout, but it is a problem for human users.
Solve this by outputting the JSON into a std::stringstream and printing
it in one go at the very end. This means JSON is accumulated in a memory
buffer, but these commands don't output a lot of JSON, so this shouldn't
be a problem.
Closesscylladb/scylladb#16216
d6d9751dd8