The timer is armed inside the section guarded by the _timer_reads_gate
therefore it has to be canceled after the gate is closed.
Otherwise we may end up with the armed timer after stop() method has
returned a ready future.
Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>
Message-Id: <1501603059-32515-1-git-send-email-vladz@scylladb.com>
(cherry picked from commit 4b28ea216d)