No more Integer object created on PartitionServiceImpl.

Java only caches Integer objects till +/- 128, so everything above that will lead
to an object being created and being disposed. This pull request changes the usage
of an ConcurrentMap with the integer as key, to an AtomicReferenceArray.

Although these objects are short lived, it is still better to not create that object
at all.
Merge pull request #1447 from pveentjer/improvement/partitionservice-unwanted-integerobjectcreation

No more Integer object created on PartitionServiceImpl.