Minor Description
Kudu master now reserve all tables/tablets' metadata in memory and disks, deleted tables and tablets were marked REMOVED/DELETED/REPLACED state but not really deleted. This could lead to huge memory usage as described in KUDU-3097.
I think it's a good idea to cleanup them in the CatalogManagerBgTasks thread. But because the data deletion tasks are done asynchronously by tablet servers, it is uncertain when metadata can be safely deleted.
Besides, we could also cleanup dead tablet servers from master's in-memory map in this thread, as I mentioned in KUDU-2915.