avoid messing with finalize cycle in Disposable.

ensure statements are closed when connection is closed

src/db/connection.cr

@@ -40,6 +40,9 @@ module DB
     abstract def build_statement(query) : Statement
 
     protected def do_close
+      @statements_cache.each do |_, stmt|
+        stmt.close
+      end
       @statements_cache.clear
     end
   end

src/db/disposable.cr

@@ -17,11 +17,6 @@ module DB
       @closed
     end
 
-    # :nodoc:
-    def finalize
-      close
-    end
-
     # Implementors overrides this method to perform resource cleanup
     protected abstract def do_close
   end