Also changed circuit breaking counter implementation to move max_count into the
picker, because this is how cluster_impl is designed. Implementation in EDS is
also modified to keep max_count in picker.
Part of C2P fallback. To support fallback to a DNS cluster.
This PR adds implementation of xds_cluster_impl_balancer, which will be responsible for circuit breaking and rpc dropping.
This PR only added RPC dropping. Circuit breaking will be done in a followup PR, after some necessary refactoring.