Efficient DataTable::sort shared with DataStore

[teh-cmc]

• Implement an efficient multi-column sorting routine for DataTable
• Make sure it follows the bucketing semantics of the store so the data is already sorted even post-bucketization
• Share as much of the code with the in-bucket sort routine

Related:

• re_datastore: properly handle already sorted data during insertion #433
• re_datastore: implement efficient reshuffling on the read path #442