No need to start a local generator.
Can just use the table's sstable generation generator
to make new sstables now that it's stateless and doesn't
depend on the highest generation found.
Note that tablet_stream_files_handler used uuid generations
unconditionally from inception
(4018dc7f0d).
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>