The sstables::sstable class has two methods for writing sstables:
1) sstable_writer get_writer(...);
2) future<> write_components(flat_mutation_reader, ...);
(1) directly exposes the writer type, so we have to update all users of
it (there is not that many) in this same patch. We defer updating
users of (2) to a follow-up commits.
105bf8888a