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.
No it won't, but neither does `getNumberOfEntries` statistic. I don't think it'd be wise for statistics to have to go to cache stores to calculate their values. They could be queried very regularly and it could hit performance badly.

Having said that, the current impl of RemoteCache.size() will first check if the stats are enabled and if so, it will calculate the sum of primary owners, otherwise it calls keySet(). The former will skip the cache store whereas the latter does not, and that's not good either.

The most efficient way to solve this, while not impacting standard stats, might be to add a new operation that does what bulk get keys does but without having to bring back all keys to client.