Currently callers of `for_each_sstable` need to
use a seastar thread to allow preemption
in the for_each_sstable loop.
Provide for_each_sstable_gently and
for_each_sstable_gently_until to make using this
facility from a coroutine easier, without requiring
a seastar thread.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>