MDEV-33763: Buffered error logging for Galera

[janlindstrom]

This is adaptation of feature by Zsolt Parragi zsolt.parragi@percona.com for Percona XtraDB Cluster to MariaDB. Original feature is documented at https://www.percona.com/blog/introducing-buffered-error-logging-in-percona-server-for-mysql/.

In this adaptation variable names are changed and integration to server code is different.

Commit adds new variables
wsrep_buffered_error_log_buffer_size ulong default 0 dynamic
Size of buffer used in buffered error logging. Because
wsrep debug setting produces large number of messages
it is recomended to set this variable around 1-100Mb.

wsrep_buffered_error_log_filename string default NULL static
  Name of the file where buffered error logging is done.

wsrep_buffered_error_log_file_size ulong default 0 dynamic
  Maximum size of one buffered error log file. This variable
  should be configured to be 10-100 times bigger than buffer
  size.

wsrep_buffered_error_log_rotations uint default 9 dynamic.
  Number of buffered error log files kept before deleting
  them.

This implementation uses logger service interface. When wsrep_buffered_error_log_buffer_size is specified and non zero, all logging is written using logger service interface. When logger buffer gets full, the server shuts down, or crashes, the buffer is written to wsrep_buffered_error_log_filename.

When log files reaches configured size limit log is rotated and only configured amount of log files are kept.

This is useful in situations where debug logs create lots of output and slow down normal server operation, but are helpful for figuring out issues, as writing to a memory buffer has less overhead.

Detailed changes:

wsrep.h
Log printing is now controlled by wsrep_debug_mode variable and
controlled by wsrep_debug and wsrep_buffered_error_log_buffer_size
variables.

sql/log.cc
Added sql_print_debug to log wsrep debug messages (WSREP_DEBUG()).

print_buffer_to_file
Actual buffered error logging if enabled will happen in this
function.

sql/mysqld.cc
Buffered error logging is initialized and shutdown here if it is
configured.

sql/signal_handler.cc
If buffered error logging is enabled remaining buffer is written
before continuing signal handling.

sql/sys_vars.cc
New variables are defined here.

sql/wsrep_buffered_error_log.cc
sql/wsrep_buffered_error_log.h
Buffered error log implementation using logger service API.

sql/wsrep_var.cc
New variables update and check functions.