Add --source option to CLI

This lets you set the source of the query for logging purposes. Tested
by running the CLI with --help to make sure the option showed up, and
added a unit test
Use UUID for native shard IDs
Work around race condition in import flow used by TestDistributedQueries
Add hash partitioning to distributed planer
Extract tupleInfos and sources in PlanFragment constructor
Move all exchange scheduling logic to SqlStageExecution
Manage output buffer set as single unit
Add hash partitioned aggregations