sstables: use time_series_sstable_set in time_window_compaction_strategy

The following experiment was performed:
1. create a TWCS table with 1 minute windows
2. fill the table with 8 windows of data
   (each window flushed to a separate sstable)
3. perform `select * from ks.t where pk = 0 limit 1` query
   with and without the change

The expectation is that with the commit, only one sstable will be opened
to fetch that one row; without the commit all 8 sstables would be opened at once.
The difference in the value of `scylla_reactor_aio_bytes_read` was measured
(value after the query minus value before the query), both with and without the commit.

With the commit, the difference was 67584.
Without the commit, the difference was 528384.
528384 / 67584 ~= 7.8.

Fixes https://github.com/scylladb/scylla/issues/6418.

sstables/sstable_set.cc

@@ -25,6 +25,7 @@
 #include "compatible_ring_position.hh"
 #include "compaction_strategy_impl.hh"
 #include "leveled_compaction_strategy.hh"
+#include "time_window_compaction_strategy.hh"
 
 #include "sstable_set_impl.hh"
 
@@ -530,6 +531,10 @@ std::unique_ptr<sstable_set_impl> leveled_compaction_strategy::make_sstable_set(
     return std::make_unique<partitioned_sstable_set>(std::move(schema));
 }
 
+std::unique_ptr<sstable_set_impl> time_window_compaction_strategy::make_sstable_set(schema_ptr schema) const {
+    return std::make_unique<time_series_sstable_set>(std::move(schema));
+}
+
 sstable_set make_partitioned_sstable_set(schema_ptr schema, lw_shared_ptr<sstable_list> all, bool use_level_metadata) {
     return sstable_set(std::make_unique<partitioned_sstable_set>(schema, use_level_metadata), schema, std::move(all));
 }

sstables/time_window_compaction_strategy.hh

@@ -184,6 +184,8 @@ public:
         return compaction_strategy_type::time_window;
     }
 
+    virtual std::unique_ptr<sstable_set_impl> make_sstable_set(schema_ptr schema) const override;
+
     virtual compaction_backlog_tracker& get_backlog_tracker() override {
         return _backlog_tracker;
     }