this series intends to deprecate `::join()`, as it always materializes a range into a concrete string. but what we always want is to print the elements in the given range to stream, or to a seastar logger, which is backed by fmtlib. also, because fmtlib offers exactly the same set of features implemented by to_string.hh, this change would allow us to use fmtlib to replace to_string.hh for better maintainability, and potentially better performance. as fmtlib is lazy evaluated, and claims to be performant under most circumstances.
Closes#13163
* github.com:scylladb/scylladb:
utils: to_string: move join to namespace utils
treewide: use fmt::join() when appropriate
row_cache: pass "const cache_entry" to operator<<
c3a2ec9d3c