xds: use previously acked version when requesting resources after stream failure

[easwars]

Expected behavior

xDS spec says the following:

Note that the version for a resource type is not a property of an individual xDS stream but rather a property of the resources themselves. If the stream becomes broken and the client creates a new stream, the client’s initial request on the new stream should indicate the most recent version seen by the client on the previous stream

Current behavior

grpc-go implementation, as part of handing stream failure and resending requests for existing subscriptions, explicitly clears the version and nonce maps. This leads to and sending out the new requests with empty version.

[easwars]

Looks like this has been the behavior from day 1: #3227