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)