Add thread-safe KuduPartitioner::PartitionRow() method

We have a scenario in IMPALA-9782 where we'd like to have multiple threads use KuduPartitioner, but this currently requires instantiating many instances, which is somewhat expensive. It would be a good improvement if we could share the KuduPartitioner. This currently doesn't work because PartitionRow() internally uses a buffer that's a member of the object - https://github.com/cloudera/kudu/blob/master/src/kudu/client/partitioner-internal.cc#L40

I talked with Alexey Serbin on slack and he suggested that adding a new method that is not stateful would be a good way to solve this (I also considered cloning KuduPartitioner).