* Add a Resize method which will be invoked by the RLS LB policy when it
receives a new cacheSize in its ServiceConfig.
* Change the size fields in the cache to int64. This is the type of the
cacheSize field in the proto.
* Store the BackoffState field in the cache.Entry by value instead of as
a pointer because it doesn not buy us much (the backoff state is not
shared between the data cache and the pending cache).