Avoid lossy LRS during cluster updates

From https://github.com/grpc/proposal/pull/200, when a cluster update for X is received and the new cluster is active after warming, the old X cluster may still be draining for an indefinite period if it has active streams. Only the stats for new requests/streams on X will participate in LRS reporting at this point. This seems to be potentially misleading when reporting load only for the new X.

There are two scenarios to consider:

1. When the EDS service name changes. I think in this case we should generate two LRS reports, one for old cluster X (including the older EDS service name) and another for new cluster X (including the new EDS service name).
2. When the EDS service name is the same. I think a unified LRS report should be created aggregating old/new cluster X.