index_reader: make the index file tracked
index_reader: make the index file tracked

Track I/O going to the index file, similarly to how we already track I/O
going to the data file.

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla