We don't explicitly cleanup the memtable, while
it might hold tokens disowned by the current node.
Flush the memtable before performing cleanup compaction
to make sure all tokens in the memtable are cleaned up.
Note that non-owned ranges are invalidate in the cache
in compaction_group::update_main_sstable_list_on_compaction_completion
using desc.ranges_for_cache_invalidation.
\Fixes #1239
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
(cherry picked from commit eb3a94e2bc)