cleanup: fix and generalize handle_manager thread

* provide an argument to the callback
* don't ignore a callback request if it's already present in the queue
* initialize mutex/cond/in_use flag before starting the thread,
  in case the first callback queueing request arrives before
  handle_manager had time to initialize
* set/check abort_manager under a mutex, otherwise handle_manager
  thread might destroy LOCK_manager before stop_handle_manager
  released it
* signal COND on queueing a callback, stop cond_wait on callback request
* always start the thread, even if flush_time is 0
* but keep the old behavior in embedded (no replication, no galera)
* style cleanups (e.g. remove volatile for a variable protected by a mutex)

sql/sql_manager.h

@@ -18,6 +18,6 @@
 
 void start_handle_manager();
 void stop_handle_manager();
-bool mysql_manager_submit(void (*action)());
+bool mysql_manager_submit(void (*action)(void *), void *data);
 
 #endif /* SQL_MANAGER_INCLUDED */