BUG#38826 Race in MYSQL_LOG::purge_logs is impossible to debug in production

BUG#39325 Server crash inside MYSQL_LOG::purge_first_log halts replicaiton

The patch reverses the order of the purging and updating events for log and relay-log.info/index files respectively.
This solves the problem of having holes caused by crashes happening between updating info/index files and purging logs.

NOTE: This is a combined patch for BUG#38826 and BUG#39325. This patch is based on bugteam tree and takes into account reviewers suggestions.

mysql-test/r/binlog_index.result

@@ -21,6 +21,7 @@ flush logs;
 *** must be a warning master-bin.000001 was not found ***
 Warnings:
 Warning	1477	Being purged log MYSQLTEST_VARDIR/log/master-bin.000001 was not found
+Warning	1477	Being purged log MYSQLTEST_VARDIR/log/master-bin.000001 was not found
 *** must show one record, of the active binlog, left in the index file after PURGE ***
 show binary logs;
 Log_name	File_size

sql/sql_class.h

@@ -205,6 +205,13 @@ class MYSQL_LOG: public TC_LOG
   time_t last_time,query_start;
   IO_CACHE log_file;
   IO_CACHE index_file;
+  /*
+    purge_temp is a temp file used in purge_logs so that the index file
+    can be updated before deleting files from disk, yielding better crash
+    recovery. It is created on demand the first time purge_logs is called
+    and then reused for subsequent calls. It is cleaned up in cleanup().
+  */
+  IO_CACHE purge_temp;
   char *name;
   char time_buff[20],db[NAME_LEN+1];
   char log_file_name[FN_REFLEN],index_file_name[FN_REFLEN];