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>

mysql-test/suite/binlog_encryption/binlog_index.result

@@ -30,6 +30,7 @@ flush logs;
 flush logs;
 *** must be a warning master-bin.000001 was not found ***
 Warnings:
+Note	1375	Binary log 'master-bin.000004' is not purged because it is the current active binlog
 Warning	1612	Being purged 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;

mysql-test/suite/binlog_encryption/binlog_xa_recover.result

@@ -93,6 +93,8 @@ master-bin.000006	#	Start_encryption	#	#
 master-bin.000006	#	Gtid_list	#	#	[#-#-#]
 master-bin.000006	#	Binlog_checkpoint	#	#	master-bin.000004
 PURGE BINARY LOGS TO "master-bin.000006";
+Warnings:
+Note	1375	Binary log 'master-bin.000004' is not purged because it may be needed for crash recovery (XID)
 show binary logs;
 Log_name	File_size
 master-bin.000004	#