database: immediately flush tables with no memtables.

If a table has no data, it may still take a long time to flush. This is
because before we even try to flush, we need go acquire a permit and
that can take a while if there is a long running flush already queued.

We can special case the situation in which there is no data in any of
the memtables owned by table and return immediately.

Signed-off-by: Glauber Costa <glauber@scylladb.com>

database.cc

@@ -1282,7 +1282,7 @@ future<> dirty_memory_manager::shutdown() {
 }
 
 future<> memtable_list::request_flush() {
-    if (!may_flush()) {
+    if (empty() || !may_flush()) {
         return make_ready_future<>();
     } else if (!_flush_coalescing) {
         _flush_coalescing = shared_promise<>();