large_data_handler: Increase verbosity on shutdown

It may hang waiting for background handlers, so it's good to know if
they exist at all

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

db/large_data_handler.cc

@@ -58,6 +58,7 @@ void large_data_handler::start() {
 future<> large_data_handler::stop() {
     if (running()) {
         _running = false;
+        large_data_logger.info("Waiting for {} background handlers", max_concurrency - _sem.available_units());
         co_await _sem.wait(max_concurrency);
     }
 }