Avoid data race in Proxier.OnUpdate

Should close #795.

The bug was that the proxier is passed as value on method decleration.
This caused a copy of Proxier to be created when the method was invoked.
The copy being a shallow copy turned out to have them both reference
a same map instance, but their mutexes were different instances.
This turned out to use different mutexes before operating on a same map
instance, which didn't make sense.