Fixes#10563
The `resourceVersion` map was doing two jobs prior to this PR. The first job was
to track what version of every resource we know envoy currently has. The
second was to track subscriptions to those resources (by way of the empty
string for a version). This mostly works out fine, but occasionally leads to
consul removing a resource and accidentally (effectively) unsubscribing at the
same time.
The fix separates these two jobs. When all of the resources for a subscription
are removed we continue to track the subscription until envoy explicitly
unsubscribes
