Mariabackup (mariadb-backup) supports the --use-memory option that
sets the buffer pool size for innodb. However, current SST scripts
do not use this option. This commit adds support for this option,
the value for which can be specified via the "use_memory" parameter
in the configuration file in the [sst], [mariabackup] or [xtrabackup]
sections (supported only for compatibility with old configurations).
In addition, if the innodb_buffer_pool_size option is specified in
the user configuration (in the main server configuration sections)
or passed to the SST scripts or the server via arguments, its value
is also passed to mariadb-backup as the value for the --use-memory
option.
A new section name [mariabackup] has also been added, which can
be used instead of the deprecated [xtrabackup] (the section name
"mariabackup" was specified in the documentation, but was not
actually supported by SST scripts before this commit).