Title: delta-xDS doesn't update the known resource version for wildcard resources

Description:
The current implementation of the delta-xDS for wildcard responses only stores the initial version for a resource name that Envoy observes, instead of the version of the latest update.
See here:

and here:

This can lead to using a stale resource without the server knowing about it.
For example:

1. Envoy receives resource_A v1 from server (updates the map to v1).
2. Envoy receives resource_A v2 from server (doesn't update the map, because it already has a resource with the same name).
3. Envoy reconnects to a new server (that only knows about v1), and publishes it has v1, although the actual resource is of v2. -> The server won't send Envoy an update until that resource is updated on the server.

Using insert_or_assign should solve the issue.