this is a part of a series to migrating from `operator<<(ostream&, ..)`
based formatting to fmtlib based formatting. the goal here is to enable
fmtlib to print `stream_reason` without the help of `operator<<`.
please note, because we still cannot use the generic formatter for
std::unordered_map provided by fmtlib, so in order to drop `operator<<`
for `stream_reason`, and to print `unordered_map<stream_reason>`,
`fmt::join()` is used as a temporary solution. we will audit all
`fmt::join()` calls, after removing the homebrew formatter of
`std::unordered_map`.
the corresponding `operator<<()` are dropped dropped in this change,
as all its callers are now using fmtlib for formatting now.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#13609
9215adee46