large_data_handler: expose methods to get threshold
large_data_handler: expose methods to get threshold

To be used for keeping large_data statistics in sstable.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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