We need to access the underlying store to use its RangeEvents function.
It is not good to use unnecessary type conversion.
The underlying store is also needed for further store upon
watchableStore.
New PR from https://github.com/coreos/etcd/pull/3575.
This add `defer` to `s.mu`. Current code does not `Unlock`
in the correct scope, I think.
(Sorry, I accidentally deleted my fork so the changes
might not sound continuous from my previous pull requests.)
