sstable: add getter for filter_tracker
sstable: add getter for filter_tracker

This will be needed to use sstable_mutation_reader for
sstable::read_row.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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