Make test helper methods protected
Update window function API to supply partitionStartPosition on reset

This reduces the amount of internal state that each window function needs to track and simplifies their logic
Add NULL values to window function unit tests
Implement graphviz printer for SampleNode
Remove dead code
Implement graphviz printer for TableCommitNode
Implement graphviz printer for TableWriterNode
Add session properties to query page in UI
Make partial aggregation memory limit configurable
Fix distributed joins

Use hash distribution, instead of hash distribution followed by
broadcast (which is just broadcast)
Add session property to turn on "big query" mode
Add session properties to query completion events