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.
@wburns Yeah, the name `getNumberOfEntriesPrimary` does imply that this method will return only the keys for which the node is a primary owner in replicated mode as well. The javadoc does explain that is not the case, but I think it would be nicer if it did.