The loop can take a long time if the number of sstables and/or ranges
are large. To fix, futurize the loop.
Fixes: #4005
Message-Id: <3b05cb84f3f57cc566702142c6365a04b075018e.1545290730.git.asias@scylladb.com>
(cherry picked from commit bcba6b4f4d)