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.
Even though the concept of primary owner is present for replicated caches, this is not relevant for statistics since it's only used for internal locking mechanisms. Code wise, it should remain as is and clarify javadoc accordingly.