The following patches convert sstable writers to use flat mutation
readers instead of the legacy mutation_reader interface.
Writers were already using flat consumer interface and used
consume_flattened_in_thread(), so most of the work was limited to
providing an appropriate equivalent for flat mutation readers.
* https://github.com/pdziepak/scylla.git flat_mutation_reader-sstable-write/v1:
flat_mutation_reader: move consumer_adapter out of consume()
flat_mutation_reader: introduce consume_in_thread()
tests/flat_mutation_reader: test consume_in_thread()
sstables: switch write_components() to flat_mutation_reader
streamed_mutation: drop streamed_mutation_returning()
sstables: convert compaction to flat_mutation_reader
mutation_reader: drop consume_flattened_in_thread()
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