commitlog::read_log_file: Preserve subscription across reading

Fixes #6265

Return type for read_log_file was previously changed from
subscription to future<>, returning the previously returned
subscriptions result of done(). But it did not preserve the
subscription itself, which in turn will cause us to (in
work::stream), call back into a deleted object.

Message-Id: <20200422090856.5218-1-calle@scylladb.com>
(cherry picked from commit 525b283326)

db/commitlog/commitlog.cc

@@ -2133,8 +2133,9 @@ db::commitlog::read_log_file(const sstring& filename, const sstring& pfx, seasta
         }).handle_exception([w](auto ep) {
             w->s.set_exception(ep);
         });
-
-        return ret.done();
+        // #6265 - must keep subscription alive.
+        auto res = ret.done();
+        return res.finally([ret = std::move(ret)] {});
     });
 }