CDK-238 deprecate partition key

Yes, it's needed since org.kitesdk.data.PartitionStrategy#partitionKeyForEntity calls 'set' on org.kitesdk.data.PartitionKey, which it inherits from org.kitesdk.data.spi.PartitionKey.

However, as Ryan pointed out I added 'set' to the public API, which isn't ideal. I'll make a change to avoid this (although it will still be protected, which is technically part of the public API, though since we are deprecating then removing PartitionKey, it's OK).