FR: support buffer pool dump /restore with mariabackup

It would be great if mariabackup could make the server create a buffer pool dump file as part of the backup process which could then be restored along with the actual backup and so provide for restoring the buffer pool contents on a restored server to warm up the buffer pool from the very start.

This would especially come in handy in combination with Galera SST, as it would lead to a newly joined node (or one that needed to be re-initialized via SST) to come up to speed with its peers rather quickly).