Loosen expected index check
Loosen expected index check

etcd behaves in somewhat unexpected way.
When etcdb places a write lock it keeps updating ttl of write lock key (something like `/foo/bar/t1/_lock_write/<uuid>`).
Then when etcdb writes the new value of the primary key (`/foo/bar/t1/1`) modifiedIndex increases not by one as I'd expect but more.
It looks like the new modifiedIndex must be higher than the write lock key's modifiedIndex.

box/etcdb,box/etcdb,box/etcdb