ISPN-4470 RemoteCache's size() method should compute size globally

Improves on the original solution in #2713 by using statistics to calculate total number of entries in the cluster, and if the statistics are disabled, it falls back retrieving the key set and getting its size.
This doesn't need to be volatile afaik since the call method is invoked on the same thread as setEnvironment.  Or is this something else?

Did you mean transient instead?