MDEV-34504 PURGE BINARY LOGS not working anymore

PURGE BINARY LOGS did not always purge binary logs. This commit fixes
some of the issues and adds notifications if a binary log cannot be
purged.

User visible changes:
- 'PURGE BINARY LOG TO log_name' and 'PURGE BINARY LOGS BEFORE date'
  worked differently. 'TO' ignored 'slave_connections_needed_for_purge'
  while 'BEFORE' did not. Now both versions ignores the
  'slave_connections_needed_for_purge variable'.
- 'PURGE BINARY LOG..' commands now returns 'note' if a binary log cannot
   be deleted like
   Note 1375 Binary log 'master-bin.000004' is not purged because it is
             the current active binlog
- Automatic binary log purges, based on date or size, will write a
  note to the error log if a binary log matching the size or date
  cannot yet be deleted.
- If 'slave_connections_needed_for_purge' is set from a config or
  command line, it is set to 0 if Galera is enabled and 1 otherwise
  (old default). This ensures that automatic binary log purge works
  with Galera as before the addition of
  'slave_connections_needed_for_purge'.
  If the variable is changed to 0, a warning will be printed to the error
  log.

Code changes:
- Added THD argument to several purge_logs related functions that needed
  THD.
- Added 'interactive' options to purge_logs functions. This allowed
  me to remove testing of sql_command == SQLCOM_PURGE.
- Changed purge_logs_before_date() to first check if log is applicable
  before calling can_purge_logs(). This ensures we do not get a
  notification for logs that does not match the remove criteria.
- MYSQL_BIN_LOG::can_purge_log() will write notifications to the user
  or error log if a log cannot yet be removed.
- log_in_use() will return reason why a binary log cannot be removed.

Changes to keep code consistent:
- Moved checking of binlog_format for Galera to be after Galera is
  initialized (The old check never worked). If Galera is enabled
  we now change the binlog_format to ROW, with a warning, instead of
  aborting the server. If this change happens a warning will be printed to
  the error log.
- Print a warning if Galera or FLASHBACK changes the binlog_format
  to ROW. Before it was done silently.

Reviewed by: Sergei Golubchik <serg@mariadb.com>,
             Kristian Nielsen <knielsen@knielsen-hq.org>

sql/sql_repl.cc

@@ -612,14 +612,26 @@ static my_bool log_in_use_callback(THD *thd, st_log_in_use *arg)
 }
 
 
-bool log_in_use(const char* log_name, uint min_connected)
+/*
+  Check if a log is in use.
+
+  @return 0  Not used
+  @return 1  A slave is reading from the log
+  @return 2  There are less than 'min_connected' slaves that
+             has recived the log.
+*/
+
+int log_in_use(const char* log_name, uint min_connected)
 {
   st_log_in_use arg;
   arg.log_name= log_name;
   arg.connected_slaves= 0;
 
-  return ((server_threads.iterate(log_in_use_callback, &arg) ||
-           arg.connected_slaves < min_connected));
+  if (server_threads.iterate(log_in_use_callback, &arg))
+    return 1;
+  if (arg.connected_slaves < min_connected)
+    return 2;
+  return 0;
 }
 
 
@@ -659,8 +671,8 @@ bool purge_master_logs(THD* thd, const char* to_log)
 
   mysql_bin_log.make_log_name(search_file_name, to_log);
   return purge_error_message(thd,
-			     mysql_bin_log.purge_logs(search_file_name, 0, 1,
-						      1, NULL));
+			     mysql_bin_log.purge_logs(thd, search_file_name,
+                                                      0, 1, 1, 1, NULL));
 }
 
 
@@ -683,7 +695,9 @@ bool purge_master_logs_before_date(THD* thd, time_t purge_time)
     return 0;
   }
   return purge_error_message(thd,
-                             mysql_bin_log.purge_logs_before_date(purge_time));
+                             mysql_bin_log.purge_logs_before_date(thd,
+                                                                  purge_time,
+                                                                  1));
 }
 
 void set_read_error(binlog_send_info *info, int error)