Incrementally select sstables from sstable set using token
in ascending order.
For leveled strategy, it returns all sstables that belong
to current interval. For other strategies, it just return
all sstables from the set.
Useful for compaction which needs all sstables that overlap
with key being currently compacted to calculate maximum
purgeable timestamp.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
sstable_set abstracts the notion of a container of sstables, allowing
different compaction strategies to supply their own implementation. The
intended user is leveled compaction strategy; since it partitions sstables,
it can quickly restrict the number of sstables that participate in a query
by looking at the min/max partition key.
sstable_set also maintains an internal lw_shared_ptr<sstable_list>,
in parallel with the abstract container. This is to support
column_family::get_sstable(), which returns a lw_shared_ptr<sstable_list>
which must be anchored somewhere if it is not saved at the caller side,
as it isn't in most current callers.
