xds_cluster_resolver: fix bug that caused us to incorrectly reuse priority child names (grpc#29174)

* xds_cluster_resolver: fix bug that caused us to incorrectly reuse priority child names

* add debug logging

* assign child policy names within each discovery mechanism, not across all mechanisms

* clang-format

* add tests

* don't need EDS service name or LOGICAL_DNS hostname in child name

* remove unnecessary code

Author: markdroth