Proposition to add support for multi commands/scripts execution for the wsrep_notify_cmd parameter

Current implementation of the wsrep_notify_cmd parameter and the --wsrep-notify-command command line argument support only a single command/script implementation of execution.

Proposition is to add support for adding multi commands/scripts to the wsrep_notify_cmd parameter for example:

• provided as a list of commands/scripts separated by commas , like wsrep_notify_cmd=script1,script2,script3 or --wsrep-notify-command="script1,script2,script3"
• multi command line arguments invocation? like --wsrep-notify-command="script1" --wsrep-notify-command="script2" --wsrep-notify-command="script3"
• multi entries in a single configuration file? (separate rows of multi wsrep_notify_cmd parameters
• join wsrep_notify_cmd parameter from separated configuration files?

Separate only by commas should be enough. This will allow to call N registered commands/scripts. Like in the software observer pattern.

It seems that the internal implementation of the mysqld should split the wsrep_notify_cmd parameter, simple loop over it and call each command/script separately. Maybe better will be to fork each script/command and join them together after the for loop. This will preserve the default behavior that invocation of the command/script block node (warning section).

Original discussion: https://github.com/MariaDB/mariadb-docker/issues/28#issuecomment-844793730