Fix scope of `Balancer.gather` `Locker`.
Fix scope of `Balancer.gather` `Locker`.

The `Locker` for `Balancer.gather` was declared by the `Balancer`
exposing it to all the balance operation functions. `drainRoot` did not
have it's own `Locker` and used this `Balancer` wide `Locker`.

See #262.

bigeasy/strata,bigeasy/strata,cesarmarinhorj/strata,cesarmarinhorj/strata