ConnectionManager: Require ILoadBalancer instead of LoadBalancer

ConnectionManager: Require ILoadBalancer instead of LoadBalancer

Since I4fdf7f7 more code stops returning the implementation, but only
returns the interface. This is a good, very welcome change. However,
ConnectionManager still requires the LoadBalancer implementation, for
no obvious reason. All code in this class works fine with the interface.

This is currently reported by Phan as a violation (and it actually is
one), e.g.:
https://integration.wikimedia.org/ci/job/mwext-php70-phan-docker/6433/console

Change-Id: I63cbb98fd277b0c64ab8b303888b9354c4be29e2