This patch makes sstable_streamed_mutation keep a reference to
sstable_data_source object which contains full state necessary to read
the sstable. That state is also shared with parent mutation_reader (only
for range queries), but now its lifetime is appropriately extended if
the mutation_reader is destoryed before streamed_mutation.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
3a27582cd4