Our "sstring_view" is an historic alias for the standard std::string_view.
The patch changes the last remaining random uses of this old alias across
our source directory to the standard type name.
After this patch, there are no more uses of the "sstring_view" alias.
It will be removed in the following patch.
Refs #4062.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>