Introduce partitioned_sstable_set
Introduce partitioned_sstable_set

partitioned_sstable_set assumes that sstable are mostly partitioned along
the token range: only a few sstables will be needed to access a particular
token.  It is implemented as an interval_map.

kjniemi/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla