Upgrade TL_WRITE_CONCURRENT_INSERT to TL_WRITE_LOW_PRIORITY if

--low-priority-updates is used and the file has holes.


Docs/manual.texi:
  Changelog
sql/sql_insert.cc:
  Fixed that sleeptime is 0 for new insert delayed threads.

include/thr_lock.h

@@ -65,6 +65,7 @@ enum thr_lock_type { TL_IGNORE=-1,
 
 extern ulong max_write_lock_count;
 extern my_bool thr_lock_inited;
+extern enum thr_lock_type thr_upgraded_concurrent_insert_lock;
 
 typedef struct st_thr_lock_data {
   pthread_t thread;