runtimevar: Fix watch not remembering last version
runtimevar: Fix watch not remembering last version

Watch didn't remember the last version, because the methods weren't
using pointers. This means every time it read the last version on the
next tick, it would be -1 and the param would be returned again. This
fix adds the pointer *s.

Change-Id: If6f43b9bc7c3790e7e1417344725a5a28de61981
Reviewed-on: https://team-review.git.corp.google.com/259508
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Tuo Shan <e38988508c7dd9b59ee1f5735040284711960df8@google.com>

google/go-cloud,google/go-cloud,google/go-cloud