Not loading more keys from mapstore than specified in max size policy. #3723. 

Fix for #3723. Not loading more keys from mapstore than specified in map's max size policy.

- Main change: MapContainer.loadInitialKeys
- Added MapServiceContext.isOwnedKey(Data) method to internal API
- Test: MaspStoreEvictionTest
- DefaultMapServiceContext now stores partitions IDs as a HashSet for lookups.