grpc: Rework ChildManager some (#2450)

1. Stop `impl`-ing `LbPolicy` - given the functionality it provided,
nothing could use it standalone anyway, since it never updated the
picker
2. Replace `resolver_update` + `ResolverUpdateSharder` with update
method that acts on `ChildUpdate`s
3. Update `GracefulSwitch` and tests accordingly
4. Add comments for readability

Author: dfawley