tombstone_gc: introduce tombstone_gc_state

and use it to access the repair history maps.

At this introductory patch, we use default-constructed
tombstone_gc_state to access the thread-local maps
temporarily and those use sites will be replaced
in following patches that will gradually pass
the tombstone_gc_state down from the compaction_manager
to where it's used.

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

db/view/view.cc

@@ -1169,7 +1169,8 @@ void view_update_builder::generate_update(clustering_row&& update, std::optional
     }
 
     auto dk = dht::decorate_key(*_schema, _key);
-    auto gc_before = ::get_gc_before_for_key(_schema, dk, _now);
+    auto gc_state = tombstone_gc_state(); // FIXME: for now
+    auto gc_before = gc_state.get_gc_before_for_key(_schema, dk, _now);
 
     // We allow existing to be disengaged, which we treat the same as an empty row.
     if (existing) {