Merge with MySQL 5.1.53

Open issues:
- A better fix for #57688; Igor is working on this
- Test failure in index_merge_innodb.test ; Igor promised to look at this
- Some Innodb tests fails (need to merge with latest xtradb) ; Kristian promised to look at this.
 - Failing tests: innodb_plugin.innodb_bug56143 innodb_plugin.innodb_bug56632 innodb_plugin.innodb_bug56680 innodb_plugin.innodb_bug57255 
- Werror is disabled;  Should be enabled after merge with xtradb.

sql/rpl_rli.h

@@ -94,6 +94,16 @@ public:
   */
   MYSQL_BIN_LOG relay_log;
   LOG_INFO linfo;
+
+  /*
+   cur_log
+     Pointer that either points at relay_log.get_log_file() or
+     &rli->cache_buf, depending on whether the log is hot or there was
+     the need to open a cold relay_log.
+
+   cache_buf 
+     IO_CACHE used when opening cold relay logs.
+   */
   IO_CACHE cache_buf,*cur_log;
 
   /* The following variables are safe to read any time */