- fix for bug #16414 (Events: Crash or hang if event drops itself)

WL#1034 (Internal CRON)
(with post-review fix)


mysql-test/r/events.result:
  update the result
mysql-test/t/events.test:
  use --sleep instead of select sleep()
  enable some scheduler related tests. They were disabled because of a hang that
  happened. The hang is fixed with this bugfix.
sql/event.cc:
  - evex_remove_from_cache reports back whether the in_memory
    object was deleted or was scheduled. In case scheduling then
    don't delete it from outside but let it do it itself
  - move out db interaction code out of evex_drop_event to db_drop_event
    so it can be called from outter space :)
sql/event_priv.h:
  - export the new function db_drop_event()
sql/event_timed.cc:
  - use db_drop_event() and don't implement the dropping ourselves
sql/sql_parse.cc:
  - some debug info about the status code returned.

sql/sql_parse.cc

@@ -3723,6 +3723,8 @@ end_with_restore_list:
         res= evex_drop_event(thd, lex->et, lex->drop_if_exists, &rows_affected);
       default:;
       }
+      DBUG_PRINT("info", ("CREATE/ALTER/DROP returned error code=%d af_rows=%d",
+                  res, rows_affected));
       if (!res)
         send_ok(thd, rows_affected);