We had once updated bbolt from v1.3.3 to v1.3.4 in #4134,
but reverted to v1.3.3 in #4156 due to "fatal error: sweep increased
allocation count" (etcd-io/bbolt#214).
The issue was fixed in bbolt v1.3.5 (etcd-io/bbolt#220).
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
