mark_for_deletion() only set an in-memory flag; the actual file
deletion ran lazily when the last shared_sstable reference dropped,
leaving a window in which a follow-up scan of the upload directory
(e.g. a second 'nodetool refresh --load-and-stream') could observe a
partially-deleted sstable and fail with malformed_sstable_exception.
Force the unlink to complete before stream() returns. For tablet
streaming, partially-contained sstables span multiple per-tablet
batches, so a defer_unlinking flag postpones the unlink until after
all sstables are streamed; for vnodes and fully-contained sstables are streamed
only once and could be removed just after being streamed.
Added a FIXME on object_storage_base::wipe and strengthened the doc on storage::wipe to
make the never-fails contract explicit
(cherry picked from commit 784127c40b)
ca9abcdcbc