Provide a tool which allows users to force manual compaction by ks.table and partition key

It's somewhat common to have a "hot partition" or otherwise "bad partition" that is causing neighbors to fail. In addition to CASSANDRA-12106, I think it would be really useful to have an optional partition key argument to nodetool compact (similar to getendpoints) which would determine which sstables a partition resides in (similar to getsstables) and then run manual compaction on just those sstables.

I know at Netflix a common way that we "mitigate" these bad partitions is to delete them, but if the partition is really large it would be nice to be able to delete them and then force compaction of sstables containing them. Obviously gc_grace get's in the way but it seems like a cheap/useful tool either way.