view: Add database getters to v._update_generator and v._builder

Both services carry database which will be used by auxiliary objects
like view_updates, view_update_builder, consumer, etc in next patches.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

db/view/view_builder.hh

@@ -189,6 +189,8 @@ public:
     static constexpr size_t batch_size = 128;
     static constexpr size_t batch_memory_max = 1024*1024;
 
+    replica::database& get_db() noexcept { return _db; }
+
 public:
     view_builder(replica::database&, db::system_keyspace&, db::system_distributed_keyspace&, service::migration_notifier&, view_update_generator& vug);
     view_builder(view_builder&&) = delete;

db/view/view_update_generator.hh

@@ -72,6 +72,8 @@ public:
     future<> stop();
     future<> register_staging_sstable(sstables::shared_sstable sst, lw_shared_ptr<replica::table> table);
 
+    replica::database& get_db() noexcept { return _db; }
+
     future<> mutate_MV(
             dht::token base_token,
             utils::chunked_vector<frozen_mutation_and_schema> view_updates,