1. merge endpoint picking and localility picking policy to one field in cluster_resolver's balancer config
- This field only supports ROUND_ROBIN or RING_HASH.
- This is to support RING_HASH policy, which is responsible both endpoint picking and locality picking.
- If policy is RING_HASH, endpoints in localities will be flattened to a list of endpoints, and passed to the policy.
1. support building policy config with RING_HASH as a child
- The config tree has one less layer comparing with ROUND_ROBIN
- This also need to define RING_HASH's balancer config config
1. Deleted test `TestEDS_UpdateSubBalancerName` because now the balancer doesn't support updating child to a custom policy.
There's a race between update sub-balancer and the first EDS resp. If
sub-balancer is updated after the first EDS resp, the old balancers
(round_robin) will create two lingering SubConns that are not handled,
which will mess up the following SubConn state updates.
Doug Fawley