The way index_reader maintains io_priority_class can be relaxed a bit. The main intent is to shorten the #13963 final patch a bit, as a side effect index_reader gets its portion of API polishing.
ref: #13963Closes#13992
* github.com:scylladb/scylladb:
index_reader: Introduce and use default arguments to constructor
index_reader: Use _pc field in get_file_input_stream_options() directly
index_reader: Move index_reader::get_file_input_stream_options to private: block