InternalTimerServiceSerializationProxy should not be serializing timers' key / namespace serializers anymore

All of the changes done to managed state surrounding how we no longer Java-serialize serializers anymore and only write the serializer snapshot, as well as how serialization compatibility checks should go through the new TypeSerializerSnapshot / TypeSerializerSchemaCompatibility interfaces, was not reflected to timers.

This was mainly due to the fact that timers were not handled by state backends (and were therefore not managed state) in the past, and were handled in an isolated manner by the InternalTimerServiceSerializationProxy.

The InternalTimerServiceSerializationProxy therefore needs to be updated accordingly.